RateLimit Fields for HTTP
draft-ietf-httpapi-ratelimit-headers-03
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| Authors | Roberto Polli , Alex Martínez Ruiz | ||
| Last updated | 2022-03-07 | ||
| Replaces | draft-polli-ratelimit-headers | ||
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draft-ietf-httpapi-ratelimit-headers-03
HTTPAPI R. Polli
Internet-Draft Team Digitale, Italian Government
Intended status: Standards Track A. Martinez
Expires: 8 September 2022 Red Hat
7 March 2022
RateLimit Fields for HTTP
draft-ietf-httpapi-ratelimit-headers-03
Abstract
This document defines the RateLimit-Limit, RateLimit-Remaining,
RateLimit-Reset fields for HTTP, thus allowing servers to publish
current service limits and clients to shape their request policy and
avoid being throttled out.
Note to Readers
_RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP working group
mailing list (httpapi@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/httpapi/
(https://mailarchive.ietf.org/arch/browse/httpapi/).
The source code and issues list for this draft can be found at
https://github.com/ietf-wg-httpapi/ratelimit-headers
(https://github.com/ietf-wg-httpapi/ratelimit-headers).
References to ThisRFC in the IANA Considerations section would be
replaced with the RFC number when assigned.
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."
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This Internet-Draft will expire on 8 September 2022.
Copyright Notice
Copyright (c) 2022 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Expressing rate-limit policies . . . . . . . . . . . . . . . 5
2.1. Time window . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Service limit . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Quota policy . . . . . . . . . . . . . . . . . . . . . . 6
3. Providing RateLimit fields . . . . . . . . . . . . . . . . . 7
3.1. Performance considerations . . . . . . . . . . . . . . . 8
4. Receiving RateLimit fields . . . . . . . . . . . . . . . . . 9
4.1. Intermediaries . . . . . . . . . . . . . . . . . . . . . 10
4.2. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Fields definition . . . . . . . . . . . . . . . . . . . . . . 10
5.1. RateLimit-Limit . . . . . . . . . . . . . . . . . . . . . 10
5.2. RateLimit-Remaining . . . . . . . . . . . . . . . . . . . 11
5.3. RateLimit-Reset . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.1. Throttling does not prevent clients from issuing
requests . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Information disclosure . . . . . . . . . . . . . . . . . 13
6.3. Remaining quota-units are not granted requests . . . . . 13
6.4. Reliability of RateLimit-Reset . . . . . . . . . . . . . 13
6.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 14
6.6. Denial of Service . . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7.1. RateLimit Parameters Registration . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Rate-limiting and quotas . . . . . . . . . . . . . . 17
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A.1. Interoperability issues . . . . . . . . . . . . . . . . . 18
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 19
B.1. Unparameterized responses . . . . . . . . . . . . . . . . 19
B.1.1. Throttling information in responses . . . . . . . . . 19
B.1.2. Use in conjunction with custom fields . . . . . . . . 20
B.1.3. Use for limiting concurrency . . . . . . . . . . . . 21
B.1.4. Use in throttled responses . . . . . . . . . . . . . 22
B.2. Parameterized responses . . . . . . . . . . . . . . . . . 22
B.2.1. Throttling window specified via parameter . . . . . . 22
B.2.2. Dynamic limits with parameterized windows . . . . . . 23
B.2.3. Dynamic limits for pushing back and slowing down . . 23
B.3. Dynamic limits for pushing back with Retry-After and slow
down . . . . . . . . . . . . . . . . . . . . . . . . . . 24
B.3.1. Missing Remaining information . . . . . . . . . . . . 25
B.3.2. Use with multiple windows . . . . . . . . . . . . . . 26
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
RateLimit fields currently used on the web . . . . . . . . . . . 30
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Since draft-ietf-httpapi-ratelimit-headers-01 . . . . . . . . . 31
Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
The widespreading of HTTP as a distributed computation protocol
requires an explicit way of communicating service status and usage
quotas.
This was partially addressed by the Retry-After header field defined
in [SEMANTICS] to be returned in 429 Too Many Requests (see
[STATUS429]) or 503 Service Unavailable responses.
Widely deployed quota mechanisms limit the number of acceptable
requests in a given time window, e.g. 10 requests per second;
currently, there is no standard way to communicate service quotas so
that the client can throttle its requests and prevent 4xx or 5xx
responses. See Appendix A for further information on the current
usage of rate limiting in HTTP.
This document defines syntax and semantics for the following fields:
* RateLimit-Limit: containing the requests quota in the time window;
* RateLimit-Remaining: containing the remaining requests quota in
the current window;
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* RateLimit-Reset: containing the time remaining in the current
window, specified in seconds.
The behavior of RateLimit-Reset is compatible with the delay-seconds
notation of Retry-After.
The fields definition allows to describe complex policies, including
the ones using multiple and variable time windows and dynamic quotas,
or implementing concurrency limits.
1.1. Goals
The goals of the RateLimit fields are:
Interoperability: Standardization of 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 fields are not meant to support
authorization or other kinds of access controls.
Throttling scope: This specification does not cover the throttling
scope, that may be the given resource-target, its parent path or
the whole Origin (see Section 7 of [RFC6454]). This can be
addressed using extensibility mechanisms such as the parameter
registry Section 7.1.
Response status code: RateLimit fields may be returned in both
successful (see Section 15.3 of [SEMANTICS]) and non-successful
responses. This specification does not cover whether non
Successful responses count on quota usage, nor it mandates any
correlation between the RateLimit values and the returned status
code.
Throttling policy: This specification does not mandate a specific
throttling policy. The values published in the fields, including
the window size, can be statically or dynamically evaluated.
Service Level Agreement: Conveyed quota hints do not imply any
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service guarantee. Server is free to throttle respectful clients
under certain circumstances.
1.2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the Augmented BNF defined in [RFC5234] and updated
by [RFC7405] along with the "#rule" extension defined in
Section 5.6.1 of [SEMANTICS].
The term Origin is to be interpreted as described in Section 7 of
[RFC6454].
This specification uses Structured Fields [SF] to specify syntax.
The terms sf-list, sf-item, sf-string, sf-token, sf-integer, bare-
item and key refer to the structured types defined therein.
2. Expressing rate-limit policies
2.1. Time window
Rate limit policies limit the number of acceptable requests in a
given time window.
A time window is expressed in seconds, using the following syntax:
time-window = delay-seconds
delay-seconds = sf-integer
Where delay-seconds is a non-negative sf-integer compatible with the
"delay-seconds" rule defined in Section 10.2.3 of [SEMANTICS].
Subsecond precision is not supported.
2.2. Service limit
The service-limit is a value associated to the maximum number of
requests that the server is willing to accept from one or more
clients on a given basis (originating IP, authenticated user,
geographical, ..) during a time-window as defined in Section 2.1.
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The service-limit is expressed in quota-units and has the following
syntax:
service-limit = quota-units
quota-units = sf-integer
where quota-units is a non-negative sf-integer.
The service-limit SHOULD match the maximum number of acceptable
requests.
The service-limit MAY differ from the total number of acceptable
requests when weight mechanisms, bursts, or other server policies are
implemented.
If the service-limit does not match the maximum number of acceptable
requests the relation with that SHOULD be communicated out-of-band.
Example: A server could
* count once requests like /books/{id}
* count twice search requests like /books?author=WuMing
so that we have the following counters
GET /books/123 ; service-limit=4, remaining: 3, status=200
GET /books?author=WuMing ; service-limit=4, remaining: 1, status=200
GET /books?author=Eco ; service-limit=4, remaining: 0, status=429
2.3. Quota policy
This specification allows describing a quota policy with the
following syntax:
quota-policy = sf-item
where the associated bare-item is a service-limit and parameters are
supported.
The following parameters are defined:
w: The REQUIRED "w" parameter specifies a time window. Its syntax
is a "time-window" defined in Section 2.1.
Other parameters are allowed and can be regarded as comments. They
ought to be registered within the "Hypertext Transfer Protocol (HTTP)
RateLimit Parameters Registry", as described in Section 7.1.
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An example policy of 100 quota-units per minute.
100;w=60
The definition of a quota-policy does not imply any specific
distribution of quota-units over time. Such service specific details
can be conveyed as parameters.
Two policy examples containing further details via custom parameters
100;w=60;comment="fixed window"
12;w=1;burst=1000;policy="leaky bucket"
To avoid clashes, implementers SHOULD prefix unregistered parameters
with an x-<vendor> identifier, e.g. x-acme-policy, x-acme-burst.
While it is useful to define a clear syntax and semantics even for
custom parameters, it is important to note that user agents are not
required to process quota policy information.
3. Providing RateLimit fields
A server uses the RateLimit response fields defined in this document
to communicate its quota policies according to the following rules:
* RateLimit-Limit and RateLimit-Reset are REQUIRED;
* RateLimit-Remaining is RECOMMENDED.
The returned values refers to the metrics used to evaluate if the
current request respects the quota policy and MAY not apply to
subsequent requests.
Example: a successful response with the following fields
RateLimit-Limit: 10
RateLimit-Remaining: 1
RateLimit-Reset: 7
does not guarantee that the next request will be successful. Server
metrics may be subject to other conditions like the one shown in the
example from Section 2.2.
A server MAY return RateLimit response fields independently of the
response status code. This includes throttled responses.
This document does not mandate any correlation between the RateLimit
values and the returned status code.
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Servers should be careful in returning RateLimit fields in
redirection responses (e.g. 3xx status codes) because a low
RateLimit-Remaining value could prevent the client from issuing
requests. For example, given the rate limiting fields below, a
client could decide to wait 10 seconds before following the Location
header, because RateLimit-Remaining is 0.
HTTP/1.1 301 Moved Permanently
Location: /foo/123
RateLimit-Remaining: 0
RateLimit-Limit: 10
RateLimit-Reset: 10
If a response contains both the Retry-After and the RateLimit-Reset
fields, the value of RateLimit-Reset SHOULD reference the same point
in time as Retry-After.
When using a policy involving more than one time-window, the server
MUST reply with the RateLimit fields related to the window with the
lower RateLimit-Remaining values.
A service returning RateLimit fields MUST NOT convey values exposing
an unwanted volume of requests and SHOULD implement mechanisms to cap
the ratio between RateLimit-Remaining and RateLimit-Reset (see
Section 6.5); this is especially important when quota-policies use a
large time-window.
Under certain conditions, a server MAY artificially lower RateLimit
field values between subsequent requests, e.g. to respond to Denial
of Service attacks or in case of resource saturation.
Servers usually establish whether the request is in-quota before
creating a response, so the RateLimit field values should be already
available in that moment. Nonetheless servers MAY decide to send the
RateLimit fields in a trailer section.
To ease the migration from existing rate limit headers, a server
SHOULD be able to provide the RateLimit-Limit field even without the
optional quota-policy section.
3.1. Performance considerations
Servers are not required to return RateLimit fields in every
response, and clients need to take this into account. For example,
an implementer concerned with performance might provide RateLimit
fields only when a given quota is going to expire.
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Implementers concerned with response fields' size, might take into
account their ratio with respect to the payload data, or use header-
compression http features such as [HPACK].
4. Receiving RateLimit fields
A client MUST process the received RateLimit fields.
A client MUST validate the values received in the RateLimit fields
before using them and check if there are significant discrepancies
with the expected ones. This includes a RateLimit-Reset moment too
far in the future or a service-limit too high.
A client receiving RateLimit fields MUST NOT assume that subsequent
responses contain the same RateLimit fields, or any RateLimit fields
at all.
Malformed RateLimit fields MAY be ignored.
A client SHOULD NOT exceed the quota-units expressed in RateLimit-
Remaining before the time-window expressed in RateLimit-Reset.
A client MAY still probe the server if the RateLimit-Reset is
considered too high.
The value of RateLimit-Reset is generated at response time: a client
aware of a significant network latency MAY behave accordingly and use
other information (e.g. the Date response header field, or otherwise
gathered metrics) to better estimate the RateLimit-Reset moment
intended by the server.
The quota-policy values and comments provided in RateLimit-Limit are
informative and MAY be ignored.
If a response contains both the RateLimit-Reset and Retry-After
fields, Retry-After MUST take precedence and RateLimit-Reset MAY be
ignored.
This specification does not mandate a specific throttling behavior
and implementers can adopt their preferred policies, including:
* slowing down or preemptively back-off their request rate when
approaching quota limits;
* consuming all the quota according to the exposed limits and then
wait.
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4.1. Intermediaries
This section documents the considerations advised in Section 16.3.2
of [SEMANTICS].
An intermediary that is not part of the originating service
infrastructure and is not aware of the quota-policy semantic used by
the Origin Server SHOULD NOT alter the RateLimit fields' values in
such a way as to communicate a more permissive quota-policy; this
includes removing the RateLimit fields.
An intermediary MAY alter the RateLimit fields in such a way as to
communicate a more restrictive quota-policy when:
* it is aware of the quota-unit semantic used by the Origin Server;
* it implements this specification and enforces a quota-policy which
is more restrictive than the one conveyed in the fields.
An intermediary SHOULD forward a request even when presuming that it
might not be serviced; the service returning the RateLimit fields is
the sole responsible of enforcing the communicated quota-policy, and
it is always free to service incoming requests.
This specification does not mandate any behavior on intermediaries
respect to retries, nor requires that intermediaries have any role in
respecting quota-policies. For example, it is legitimate for a proxy
to retransmit a request without notifying the client, and thus
consuming quota-units.
4.2. Caching
As is the ordinary case for HTTP caching ([RFC7234]), a response with
RateLimit fields might be cached and re-used for subsequent requests.
A cached RateLimit response does not modify quota counters but could
contain stale information. Clients interested in determining the
freshness of the RateLimit fields could rely on fields such as Date
and on the time-window of a quota-policy.
5. Fields definition
The following RateLimit response fields are defined
5.1. RateLimit-Limit
The RateLimit-Limit response field indicates the service-limit
associated to the client in the current time-window.
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If the client exceeds that limit, it MAY not be served.
The field is a List Structured Field of positive length. The first
member is named expiring-limit and its syntax is service-limit, while
the syntax of the other optional members is quota-policy
RateLimit-Limit = sf-list
The expiring-limit value MUST be set to the service-limit that is
closer to reach its limit.
The quota-policy is defined in Section 2.3, and its values are
informative.
RateLimit-Limit: 100
A time-window associated to expiring-limit can be communicated via an
optional quota-policy value, like shown in the following example
RateLimit-Limit: 100, 100;w=10
If the expiring-limit is not associated to a time-window, the time-
window MUST either be:
* inferred by the value of RateLimit-Reset at the moment of the
reset, or
* communicated out-of-band (e.g. in the documentation).
Policies using multiple quota limits MAY be returned using multiple
quota-policy items, like shown in the following two examples:
RateLimit-Limit: 10, 10;w=1, 50;w=60, 1000;w=3600, 5000;w=86400
RateLimit-Limit: 10, 10;w=1;burst=1000, 1000;w=3600
This field MUST NOT occur multiple times and can be sent in a trailer
section.
5.2. RateLimit-Remaining
The RateLimit-Remaining response field indicates the remaining quota-
units defined in Section 2.2 associated to the client.
The field is an Integer Structured Field and its value is
RateLimit-Remaining = quota-units
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This field MUST NOT occur multiple times and can be sent in a trailer
section.
Clients MUST NOT assume that a positive RateLimit-Remaining value is
a guarantee that further requests will be served.
A low RateLimit-Remaining value is like a yellow traffic-light for
either the number of requests issued in the time-window or the
request throughput: the red light may arrive suddenly (see
Section 3).
One example of RateLimit-Remaining use is below.
RateLimit-Remaining: 50
5.3. RateLimit-Reset
The RateLimit-Reset response field indicates either
* the number of seconds until the quota resets.
The field is an Integer Structured Field and its value is
RateLimit-Reset = delay-seconds
The delay-seconds format is used because:
* it does not rely on clock synchronization and is resilient to
clock adjustment and clock skew between client and server (see
Section 5.6.7 of [SEMANTICS]);
* it mitigates the risk related to thundering herd when too many
clients are serviced with the same timestamp.
This field MUST NOT occur multiple times and can be sent in a trailer
section.
An example of RateLimit-Reset use is below.
RateLimit-Reset: 50
The client MUST NOT assume that all its service-limit will be
restored after the moment referenced by RateLimit-Reset. The server
MAY arbitrarily alter the RateLimit-Reset value between subsequent
requests e.g. in case of resource saturation or to implement sliding
window policies.
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6. Security Considerations
6.1. Throttling does not prevent clients from issuing requests
This specification does not prevent clients to make over-quota
requests.
Servers should always implement mechanisms to prevent resource
exhaustion.
6.2. 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 2xx responses
consume quota, if 401 and 403 responses count on quota a malicious
client could probe the endpoint to get traffic information of another
user.
As intermediaries might retransmit requests and consume quota-units
without prior knowledge of the User Agent, RateLimit fields might
reveal the existence of an intermediary to the User Agent.
6.3. Remaining quota-units are not granted requests
RateLimit-* fields convey hints from the server to the clients in
order to avoid being throttled out.
Clients MUST NOT consider the quota-units returned in RateLimit-
Remaining as a service level agreement.
In case of resource saturation, the server MAY artificially lower the
returned values or not serve the request regardless of the advertised
quotas.
6.4. Reliability of RateLimit-Reset
Consider that service-limit may not be restored after the moment
referenced by RateLimit-Reset, and the RateLimit-Reset value should
not be considered fixed nor constant.
Subsequent requests may return a higher RateLimit-Reset value to
limit concurrency or implement dynamic or adaptive throttling
policies.
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6.5. Resource exhaustion
When returning RateLimit-Reset you must be aware that many throttled
clients may come back at the very moment specified.
This is true for Retry-After too.
For example, if the quota resets every day at 18:00:00 and your
server returns the RateLimit-Reset accordingly
Date: Tue, 15 Nov 1994 08:00:00 GMT
RateLimit-Reset: 36000
there's a high probability that all clients will show up at 18:00:00.
This could be mitigated by adding some jitter to the field-value.
Resource exhaustion issues can be associated with quota policies
using a large time-window, because a user agent by chance or on
purpose might consume most of its quota-units in a significantly
shorter interval.
This behavior can be even triggered by the provided RateLimit fields.
The following example describes a service with an unconsumed quota-
policy of 10000 quota-units per 1000 seconds.
RateLimit-Limit: 10000, 10000;w=1000
RateLimit-Remaining: 10000
RateLimit-Reset: 10
A client implementing a simple ratio between RateLimit-Remaining and
RateLimit-Reset could infer an average throughput of 1000 quota-units
per second, while RateLimit-Limit conveys a quota-policy with an
average of 10 quota-units per second. If the service cannot handle
such load, it should return either a lower RateLimit-Remaining value
or an higher RateLimit-Reset value. Moreover, complementing large
time-window quota-policies with a short time-window one mitigates
those risks.
6.6. Denial of Service
RateLimit fields may assume unexpected values by chance or purpose.
For example, an excessively high RateLimit-Remaining value may be:
* used by a malicious intermediary to trigger a Denial of Service
attack or consume client resources boosting its requests;
* passed by a misconfigured server;
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or an high RateLimit-Reset value could inhibit clients to contact the
server.
Clients MUST validate the received values to mitigate those risks.
7. 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 ([SEMANTICS]):
+=====================+===========+========================+
| Field Name | Status | Specification |
+=====================+===========+========================+
| RateLimit-Limit | permanent | Section 5.1 of ThisRFC |
+---------------------+-----------+------------------------+
| RateLimit-Remaining | permanent | Section 5.2 of ThisRFC |
+---------------------+-----------+------------------------+
| RateLimit-Reset | permanent | Section 5.3 of ThisRFC |
+---------------------+-----------+------------------------+
Table 1
7.1. RateLimit Parameters Registration
IANA is requested to create a new registry to be called "Hypertext
Transfer Protocol (HTTP) RateLimit Parameters Registry", to be
located at https://www.iana.org/assignments/http-ratelimit-parameters
(https://www.iana.org/assignments/http-ratelimit-parameters).
Registration is done on the advice of a Designated Expert, appointed
by the IESG or their delegate. All entries are Specification
Required ([IANA], Section 4.6).
Registration requests consist of the following information:
* Parameter name: The parameter name, conforming to [SF].
* Field name: The RateLimit field for which the parameter is
registered. If a parameter is intended to be used with multiple
fields, it has to be registered for each one.
* Description: A brief description of the parameter.
* Specification document: A reference to the document that specifies
the parameter, preferably including a URI that can be used to
retrieve a copy of the document.
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* Comments (optional): Any additional information that can be
useful.
The initial contents of this registry should be:
+=================+=========+============+===============+==========+
| Field Name |Parameter|Description |Specification |Comments |
| |name | | |(optional)|
+=================+=========+============+===============+==========+
| RateLimit-Limit |w |Time window |Section 2.3 of | |
| | | |ThisRFC | |
+-----------------+---------+------------+---------------+----------+
Table 2
8. References
8.1. Normative References
[IANA] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/rfc/rfc6454>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
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[SEMANTICS]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>.
[SF] Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.
8.2. Informative References
[HPACK] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/rfc/rfc7541>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/rfc/rfc3339>.
[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/rfc/rfc6585>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/rfc/rfc7234>.
[STATUS429]
Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, DOI 10.17487/RFC6525, February 2012,
<https://www.rfc-editor.org/rfc/rfc6525>.
[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.
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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.
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User Agents interfacing with different servers may thus need to
process different headers, or the very same application interface
that sits behind different reverse proxies may reply with different
throttling headers.
Appendix B. Examples
B.1. Unparameterized responses
B.1.1. Throttling information in responses
The client exhausted its service-limit for the next 50 seconds. The
time-window is communicated out-of-band or inferred by the field
values.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
Ratelimit-Remaining: 0
Ratelimit-Reset: 50
{"hello": "world"}
Since the field values are not necessarily correlated with the
response status code, a subsequent request is not required to fail.
The example below shows that the server decided to serve the request
even if RateLimit-Remaining is 0. Another server, or the same server
under other load conditions, could have decided to throttle the
request instead.
Request:
GET /items/456 HTTP/1.1
Host: api.example
Response:
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HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
Ratelimit-Remaining: 0
Ratelimit-Reset: 48
{"still": "successful"}
B.1.2. Use in conjunction with custom fields
The server uses two custom fields, namely acme-RateLimit-DayLimit and
acme-RateLimit-HourLimit to expose the following policy:
* 5000 daily quota-units;
* 1000 hourly quota-units.
The client consumed 4900 quota-units in the first 14 hours.
Despite the next hourly limit of 1000 quota-units, the closest limit
to reach is the daily one.
The server then exposes the RateLimit-* fields to inform the client
that:
* it has only 100 quota-units left;
* the window will reset in 10 hours.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
acme-RateLimit-DayLimit: 5000
acme-RateLimit-HourLimit: 1000
RateLimit-Limit: 5000
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"}
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B.1.3. Use for limiting concurrency
Throttling fields may be used to limit concurrency, advertising
limits that are lower than the usual ones in case of saturation, thus
increasing availability.
The server adopted a basic policy of 100 quota-units per minute, and
in case of resource exhaustion adapts the returned values reducing
both RateLimit-Limit and RateLimit-Remaining.
After 2 seconds the client consumed 40 quota-units
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 60
RateLimit-Reset: 58
{"elapsed": 2, "issued": 40}
At the subsequent request - due to resource exhaustion - the server
advertises only RateLimit-Remaining: 20.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 20
RateLimit-Reset: 56
{"elapsed": 4, "issued": 41}
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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-Reset
reference the same moment, but this is not a requirement.
The 429 Too Many Requests HTTP status code is just used as an
example.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 429 Too Many Requests
Content-Type: application/json
Date: Mon, 05 Aug 2019 09:27:00 GMT
Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
RateLimit-Reset: 5
RateLimit-Limit: 100
Ratelimit-Remaining: 0
{
"title": "Too Many Requests",
"status": 429,
"detail": "You have exceeded your quota"
}
B.2. Parameterized responses
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:
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HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100, 100;w=60
Ratelimit-Remaining: 99
Ratelimit-Reset: 50
{"hello": "world"}
B.2.2. Dynamic limits with parameterized windows
The policy conveyed by RateLimit-Limit states that the server accepts
100 quota-units per minute.
To avoid resource exhaustion, the server artificially lowers the
actual limits returned in the throttling headers.
The RateLimit-Remaining then advertises only 9 quota-units for the
next 50 seconds to slow down the client.
Note that the server could have lowered even the other values in
RateLimit-Limit: this specification does not mandate any relation
between the field values contained in subsequent responses.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10, 100;w=60
Ratelimit-Remaining: 9
Ratelimit-Reset: 50
{
"status": 200,
"detail": "Just slow down without waiting."
}
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 RateLimit-Remaining:
0 for the next 20 seconds.
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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-Limit: 0, 15;w=20
Ratelimit-Remaining: 0
Ratelimit-Reset: 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:
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HTTP/1.1 429 Too Many Requests
Content-Type: application/json
Retry-After: 20
RateLimit-Limit: 15, 100;w=60
Ratelimit-Remaining: 15
Ratelimit-Reset: 40
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
Note that in this last response the client is expected to honor
Retry-After and perform no requests for the specified amount of time,
whereas the previous example would not force the client to stop
requests before the reset time is elapsed, as it would still be free
to query again the server even if it is likely to have the request
rejected.
B.3.1. Missing Remaining information
The server does not expose RateLimit-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 couple RateLimit-Limit and RateLimit-Reset.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"first": "request"}
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
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HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 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 fields to inform the client
that:
* it has only 100 quota-units left;
* the window will reset in 10 hours;
* the expiring-limit is 5000.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 OK
Content-Type: application/json
RateLimit-Limit: 5000, 1000;w=3600, 5000;w=86400
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"}
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FAQ
_RFC Editor: Please remove this section before publication._
1. Why defining standard fields for throttling?
To simplify enforcement of throttling policies.
2. Can I use RateLimit-* in throttled responses (eg with status code
429)?
Yes, you can.
3. Are those specs tied to RFC 6585?
No. [RFC6585] defines the 429 status code and we use it just as
an example of a throttled request, that could instead use even
403 or whatever status code. The goal of this specification is
to standardize the name and semantic of three ratelimit fields
widely used on the internet. Stricter relations with status
codes or error response payloads would impose behaviors to all
the existing implementations making the adoption more complex.
4. Why don't pass the throttling scope as a parameter?
The word "scope" can have different meanings: for example it can
be an URL, or an authorization scope. Since authorization is out
of the scope of this document (see Section 1.1), and that we rely
only on [SEMANTICS], in Section 1.1 we defined "scope" in terms
of URL.
Since clients are not required to process quota policies (see
Section 4), we could add a new "RateLimit-Scope" field to this
spec. See this discussion on a similar thread
(https://github.com/httpwg/http-core/pull/317#issuecomment-
585868767)
Specific ecosystems can still bake their own prefixed parameters,
such as acme-auth-scope or acme-url-scope and ensure that clients
process them. This behavior cannot be relied upon when
communicating between different ecosystems.
We are open to suggestions: comment on this issue
(https://github.com/ioggstream/draft-polli-ratelimit-headers/
issues/70)
5. Why using delay-seconds instead of a UNIX Timestamp? Why not
using subsecond precision?
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Using delay-seconds aligns with Retry-After, which is returned in
similar contexts, eg on 429 responses.
Timestamps require a clock synchronization protocol (see
Section 5.6.7 of [SEMANTICS]). 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. Why not support multiple quota remaining?
While this might be of some value, my experience suggests that
overly-complex quota implementations results in lower
effectiveness of this policy. This spec allows the client to
easily focusing on RateLimit-Remaining and RateLimit-Reset.
7. Shouldn't I limit concurrency instead of request rate?
You can use this specification to limit concurrency at the HTTP
level (see {#use-for-limiting-concurrency}) and help clients to
shape their requests avoiding being throttled out.
A problematic way to limit concurrency is connection dropping,
especially when connections are multiplexed (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.
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8. Do a positive value of RateLimit-Remaining imply any service
guarantee for my future requests to be served?
No. FAQ integrated in Section 5.2.
9. Is the quota-policy definition Section 2.3 too complex?
You can always return the simplest form of the 3 fields
RateLimit-Limit: 100
RateLimit-Remaining: 50
RateLimit-Reset: 60
The key runtime value is the first element of the list: expiring-
limit, the others quota-policy are informative. So for the following
field:
RateLimit-Limit: 100, 100;w=60;burst=1000;comment="sliding window", 5000;w=3600;burst=0;comment="fixed window"
the key value is the one referencing the lowest limit: 100
1. Can we use shorter names? Why don't put everything in one field?
The most common syntax we found on the web is X-RateLimit-* and when
starting this I-D we opted for it (https://github.com/ioggstream/
draft-polli-ratelimit-headers/issues/34#issuecomment-519366481)
The basic form of those fields is easily parseable, even by
implementers processing responses using technologies like dynamic
interpreter with limited syntax.
Using a single field complicates parsing and takes a significantly
different approach from the existing ones: this can limit adoption.
1. Why don't mention connections?
Beware of the term "connection":   - it is just
_one_ possible saturation cause. Once you go that path 
you will expose other infrastructural details (bandwidth, CPU, ..
see Section 6.2)  and complicate client compliance;
 - it is an infrastructural detail defined in terms of
server and network  rather than the consumed service.
This specification protects the services first, and then the
infrastructures through client cooperation (see Section 6.1).
  RateLimit fields enable sending _on the same
connection_ different limit values  on each response,
depending on the policy scope (e.g. per-user, per-custom-key, ..)

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2. Can intermediaries alter RateLimit fields?
Generally, they should not because it might result in unserviced
requests. There are reasonable use cases for intermediaries
mangling RateLimit fields though, e.g. when they enforce stricter
quota-policies, or when they are an active component of the
service. In those case we will consider them as part of the
originating infrastructure.
3. Why the w parameter is just informative? Could it be used by a
client to determine the request rate?
A non-informative w parameter might be fine in an environment
where clients and servers are tightly coupled. Conveying
policies with this detail on a large scale would be very complex
and implementations would be likely not interoperable. We thus
decided to leave w as an informational parameter and only rely on
RateLimit-Limit, RateLimit-Remaining and RateLimit-Reset for
defining the throttling behavior.
RateLimit fields currently used on the web
_RFC Editor: Please remove this section before publication._
Commonly used header field names are:
* X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset;
* X-Rate-Limit-Limit, X-Rate-Limit-Remaining, X-Rate-Limit-Reset.
There are variants too, where the window is specified in the header
field name, eg:
* x-ratelimit-limit-minute, x-ratelimit-limit-hour, x-ratelimit-
limit-day
* x-ratelimit-remaining-minute, x-ratelimit-remaining-hour, x-
ratelimit-remaining-day
Here are some interoperability issues:
* X-RateLimit-Remaining references different values, depending on
the implementation:
- seconds remaining to the window expiration
- milliseconds remaining to the window expiration
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- seconds since UTC, in UNIX Timestamp [UNIX]
- a datetime, either IMF-fixdate [SEMANTICS] or [RFC3339]
* different headers, with the same semantic, are used by different
implementers:
- X-RateLimit-Limit and X-Rate-Limit-Limit
- X-RateLimit-Remaining and X-Rate-Limit-Remaining
- X-RateLimit-Reset and X-Rate-Limit-Reset
The semantic of RateLimit-Remaining depends on the windowing
algorithm. A sliding window policy for example may result in having
a RateLimit-Remaining value related to the ratio between the current
and the maximum throughput. e.g.
RateLimit-Limit: 12, 12;w=1
RateLimit-Remaining: 6 ; using 50% of throughput, that is 6 units/s
RateLimit-Reset: 1
If this is the case, the optimal solution is to achieve
RateLimit-Limit: 12, 12;w=1
RateLimit-Remaining: 1 ; using 100% of throughput, that is 12 units/s
RateLimit-Reset: 1
At this point you should stop increasing your request rate.
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
_RFC Editor: Please remove this section before publication._
Since draft-ietf-httpapi-ratelimit-headers-01
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* Update IANA considerations #60
* Use Structured fields #58
* Reorganize document #67
Since draft-ietf-httpapi-ratelimit-headers-00
* Use I-D.httpbis-semantics, which includes referencing delay-
seconds instead of delta-seconds. #5
Authors' Addresses
Roberto Polli
Team Digitale, Italian Government
Italy
Email: robipolli@gmail.com
Alejandro Martinez Ruiz
Red Hat
Email: amr@redhat.com
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