HTTPAPI Working Group R. Polli
Internet-Draft Team Digitale, Italian Government
Intended status: Standards Track A. Martinez
Expires: June 25, 2023 Red Hat
December 22, 2022
RateLimit Fields for HTTP
draft-ietf-httpapi-ratelimit-headers-06
Abstract
This document defines the RateLimit-Limit, RateLimit-Remaining,
RateLimit-Reset and RateLimit-Policy HTTP fields for servers to
advertise their current service rate limits, thereby allowing clients
to avoid being throttled.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Quota Policy . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Time Window . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Service Limit . . . . . . . . . . . . . . . . . . . . . . 6
3. RateLimit Field Definitions . . . . . . . . . . . . . . . . . 7
3.1. RateLimit-Limit . . . . . . . . . . . . . . . . . . . . . 7
3.2. RateLimit-Policy . . . . . . . . . . . . . . . . . . . . 7
3.3. RateLimit-Remaining . . . . . . . . . . . . . . . . . . . 8
3.4. RateLimit-Reset . . . . . . . . . . . . . . . . . . . . . 8
4. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Performance Considerations . . . . . . . . . . . . . . . 10
5. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Intermediaries . . . . . . . . . . . . . . . . . . . . . 11
5.2. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6.1. Throttling does not prevent clients from issuing requests 12
6.2. Information disclosure . . . . . . . . . . . . . . . . . 12
6.3. Remaining quota units are not granted requests . . . . . 13
6.4. Reliability of RateLimit-Reset . . . . . . . . . . . . . 13
6.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 13
6.5.1. Denial of Service . . . . . . . . . . . . . . . . . . 14
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
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8.1. RateLimit Parameters Registration . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 17
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Appendix A. Rate-limiting and quotas . . . . . . . . . . . . . . 18
A.1. Interoperability issues . . . . . . . . . . . . . . . . . 19
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 . . . . . . . . . . . . . . . . . 23
B.2.1. Throttling window specified via parameter . . . . . . 23
B.2.2. Dynamic limits with parameterized windows . . . . . . 23
B.2.3. Dynamic limits for pushing back and slowing down . . 24
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 . . . . . . . . . . . . . . . . . . . . . . . . 32
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
F.1. Since draft-ietf-httpapi-ratelimit-headers-03 . . . . . . 32
F.2. Since draft-ietf-httpapi-ratelimit-headers-02 . . . . . . 32
F.3. Since draft-ietf-httpapi-ratelimit-headers-01 . . . . . . 32
F.4. Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
Rate limiting 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 its requests to prevent errors. This
document defines a set of standard HTTP fields to enable rate
limiting:
o RateLimit-Limit: the server's quota for requests by the client in
the time window,
o RateLimit-Remaining: the remaining quota in the current window,
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o RateLimit-Reset: the time remaining in the current window,
specified in seconds, and
o RateLimit-Policy: the quota policy.
These fields allow the establishment of complex rate limiting
policies, including using multiple and variable time windows and
dynamic quotas, and implementing concurrency limits.
The behavior of the RateLimit-Reset field is compatible with the
delay-seconds notation of Retry-After.
1.1. Goals
The goals of this document 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 [WEB-ORIGIN]). This can be
addressed using extensibility mechanisms such as the parameter
registry Section 8.1.
Response status code: RateLimit 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 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.
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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.
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 [HTTP].
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".
The fields defined in this document are collectively referred to as
"RateLimit fields".
2. Concepts
2.1. Quota Policy
A quota policy is described in terms of quota units (Section 2.3) and
a time window (Section 2.2). It is an Item whose bare item is a
service limit (Section 2.3), along with associated Parameters.
The following parameters are defined in this specification:
w: The REQUIRED "w" parameter value conveys a time window value as
defined in Section 2.2.
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 8.1.
For example, a quota policy of 100 quota units per minute:
100;w=60
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The definition of a quota policy does not imply any specific
distribution of quota units within the time window. If applicable,
these details can be conveyed as extension parameters.
For example, two quota policies containing further details via
extension parameters:
100;w=60;comment="fixed window"
12;w=1;burst=1000;policy="leaky bucket"
To avoid clashes, implementers SHOULD prefix unregistered parameters
with a vendor identifier, e.g. "acme-policy", "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.
2.2. Time Window
Rate limit policies limit the number of acceptable requests within a
given time interval, known as a time window.
The time window is a non-negative Integer value expressing that
interval in seconds, similar to the "delay-seconds" rule defined in
Section 10.2.3 of [HTTP]. Subsecond precision is not supported.
2.3. Service Limit
The service limit is associated with 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 (Section 2.2).
The service limit is a non-negative Integer expressed in quota units.
The service limit SHOULD match the maximum number of acceptable
requests. However, 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
o count once requests like "/books/{id}"
o count twice search requests like "/books?author=WuMing"
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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
3. RateLimit Field Definitions
The following RateLimit response fields are defined.
3.1. RateLimit-Limit
The "RateLimit-Limit" response field indicates the service limit
(Section 2.3) associated with the client in the current time window
(Section 2.2). If the client exceeds that limit, it MAY not be
served.
The field is an Item and its value is a non-negative Integer referred
to as the "expiring-limit". This specification does not define
Parameters for this field. If they appear, they MUST be ignored.
The expiring-limit MUST be set to the service limit that is closest
to reaching its limit, and the associated time window MUST either be:
o inferred by the value of RateLimit-Reset field at the moment of
the reset, or
o communicated out-of-band (e.g. in the documentation).
The RateLimit-Policy field (see Section 3.2), might contain
information on the associated time window.
RateLimit-Limit: 100
This field can be sent in a trailer section.
3.2. RateLimit-Policy
The "RateLimit-Policy" response field indicates the quota policies
currently associated with the client. Its value is informative.
The field is a non-empty List of Items. Each item is a quota policy
(Section 2.1).
This field can convey the time window associated with the expiring-
limit, as shown in this example:
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RateLimit-Policy: 100;w=10
RateLimit-Limit: 100
These examples show multiple policies being returned:
RateLimit-Policy: 10;w=1, 50;w=60, 1000;w=3600, 5000;w=86400
RateLimit-Policy: 10;w=1;burst=1000, 1000;w=3600
This field can be sent in a trailer section.
3.3. RateLimit-Remaining
The "RateLimit-Remaining" response field indicates the remaining
quota units associated to the expiring-limit.
The field is an Item and its value is a non-negative Integer
expressed in quota units (Section 2.3). This specification does not
define Parameters for this field. If they appear, they MUST be
ignored.
This field can be sent in a trailer section.
Clients MUST NOT assume that a positive RateLimit-Remaining field
value is a guarantee that further requests will be served.
When the value of RateLimit-Remaining is low, it indicates that the
server may soon throttle the client (see Section 4).
For example:
RateLimit-Remaining: 50
3.4. RateLimit-Reset
The "RateLimit-Reset" field response field indicates the number of
seconds until the quota associated to the expiring-limit resets.
The field is a non-negative Integer compatible with the delay-seconds
rule, because:
o 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]);
o it mitigates the risk related to thundering herd when too many
clients are serviced with the same timestamp.
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This specification does not define Parameters for this field. If
they appear, they MUST be ignored.
This field can be sent in a trailer section.
An example of RateLimit-Reset field use is below.
RateLimit-Reset: 50
The client MUST NOT assume that all its service limit will be reset
at the moment indicated by the RateLimit-Reset field. The server MAY
arbitrarily alter the RateLimit-Reset field value between subsequent
requests; for example, in case of resource saturation or to implement
sliding window policies.
4. Server Behavior
A server uses the RateLimit fields to communicate its quota policies.
Sending the RateLimit-Limit and RateLimit-Reset fields is REQUIRED;
sending RateLimit-Remaining field is RECOMMENDED.
A server MAY return RateLimit fields independently of the response
status code. This includes on throttled responses. This document
does not mandate any correlation between the RateLimit field values
and the returned status code.
Servers should be careful when returning RateLimit fields in
redirection responses (i.e., responses with 3xx status codes) because
a low RateLimit-Remaining field value could prevent the client from
issuing requests. For example, given the RateLimit 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
RateLimit-Remaining field value 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 RateLimit-Reset field value SHOULD reference the same
point in time as the Retry-After field value.
When using a policy involving more than one time window, the server
MUST reply with the RateLimit fields related to the time window with
the lower RateLimit-Remaining field values.
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A service using 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 field
values (see Section 6.5); this is especially important when a quota
policy uses 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.
4.1. Performance Considerations
Servers are not required to return RateLimit fields in every
response, and clients need to take this into account. For example,
an implementer concerned with performance might provide RateLimit
fields only when a given quota is going to expire.
Implementers concerned with response fields' size, might take into
account their ratio with respect to the content length, or use
header-compression HTTP features such as [HPACK].
5. Client Behavior
The RateLimit 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 they will succeed.
For 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.
Servers' behavior may be subject to other conditions like the one
shown in the example from Section 2.3.
A client MUST validate the RateLimit fields before using them and
check if there are significant discrepancies with the expected ones.
This includes a RateLimit-Reset field moment too far in the future
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(e.g. similarly to receiving "Retry-after: 1000000") or a service-
limit too high.
A client receiving RateLimit fields MUST NOT assume that future
responses will contain the same RateLimit fields, or any RateLimit
fields at all.
Malformed RateLimit fields MUST be ignored.
A client SHOULD NOT exceed the quota units conveyed by the RateLimit-
Remaining field before the time window expressed in RateLimit-Reset
field.
A client MAY still probe the server if the RateLimit-Reset field is
considered too high.
The value of RateLimit-Reset field 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
field moment intended by the server.
The details provided in RateLimit-Policy field are informative and
MAY be ignored.
If a response contains both the RateLimit-Reset and Retry-After
fields, the Retry-After field MUST take precedence and the RateLimit-
Reset field MAY be ignored.
This specification does not mandate a specific throttling behavior
and implementers can adopt their preferred policies, including:
o slowing down or preemptively back-off their request rate when
approaching quota limits;
o consuming all the quota according to the exposed limits and then
wait.
5.1. 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 fields' values in
such a way as to communicate a more permissive quota policy; this
includes removing the RateLimit fields.
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An intermediary MAY alter the RateLimit fields in such a way as to
communicate a more restrictive quota policy when:
o it is aware of the quota unit semantic used by the Origin Server;
o 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.
Privacy considerations (Section 7) provide further guidance on
intermediaries.
5.2. Caching
[HTTP-CACHING] defines how responses can be stored and reused for
subsequent requests, including those with RateLimit fields. Because
the information in RateLimit 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]).
6. Security Considerations
6.1. 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.
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-successful
responses consume quota, if error responses (such as 401
(Unauthorized) and 403 (Forbidden)) count against quota, a malicious
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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 help them avoid being throttled out.
Clients MUST NOT consider the quota units (Section 2.3) returned in
RateLimit-Remaining field 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 might not be restored after the moment
referenced by RateLimit-Reset field, and the RateLimit-Reset field
value may not be fixed nor constant.
Subsequent requests might return a higher RateLimit-Reset field value
to limit concurrency or implement dynamic or adaptive throttling
policies.
6.5. Resource exhaustion
When returning RateLimit-Reset field 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 field 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.
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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
RateLimit-Policy: 10000;w=1000
RateLimit-Remaining: 10000
RateLimit-Reset: 10
A client implementing a simple ratio between RateLimit-Remaining
field and RateLimit-Reset field could infer an average throughput of
1000 quota units per second, while the RateLimit-Limit field 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 field value or an higher RateLimit-Reset field
value. Moreover, complementing large time window quota policies with
a short time window one mitigates those risks.
6.5.1. Denial of Service
RateLimit fields may contain unexpected values by chance or on
purpose. For example, an excessively high RateLimit-Remaining field
value may be:
o used by a malicious intermediary to trigger a Denial of Service
attack or consume client resources boosting its requests;
o passed by a misconfigured server;
or a high RateLimit-Reset field value could inhibit clients to
contact the server.
Clients MUST validate the received values to mitigate those risks.
7. 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).
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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 fields can define
specific techniques to mitigate the risks of re-identification.
8. 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-Limit | permanent | Section 3.1 of RFC nnnn |
| RateLimit-Remaining | permanent | Section 3.3 of RFC nnnn |
| RateLimit-Reset | permanent | Section 3.4 of RFC nnnn |
| RateLimit-Policy | permanent | Section 3.2 of RFC nnnn |
+---------------------+-----------+-------------------------+
8.1. 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
[1]. 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:
o Parameter name: The parameter name, conforming to
[STRUCTURED-FIELDS].
o 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.
o Description: A brief description of the parameter.
o 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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o Comments (optional): Any additional information that can be
useful.
The initial contents of this registry should be:
+----------------+----------+------------+--------------+-----------+
| Field Name | Paramete | Descriptio | Specificatio | Comments |
| | r name | n | n | (optional |
| | | | | ) |
+----------------+----------+------------+--------------+-----------+
| RateLimit- | w | Time | Section 2.1 | |
| Policy | | window | of RFC nnnn | |
+----------------+----------+------------+--------------+-----------+
9. References
9.1. 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,
.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
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[STRUCTURED-FIELDS]
Nottingham, M. and P-H. 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,
.
9.2. 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.
9.3. URIs
[1] https://www.iana.org/assignments/http-ratelimit-parameters
[2] https://github.com/httpwg/http-core/
pull/317#issuecomment-585868767
[3] https://github.com/ioggstream/draft-polli-ratelimit-headers/
issues/70
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[4] https://community.ntppool.org/t/another-ntp-client-failure-
story/1014/
[5] https://lists.w3.org/Archives/Public/ietf-http-
wg/2019JulSep/0202.html
[6] https://github.com/ioggstream/draft-polli-ratelimit-headers/
issues/34#issuecomment-519366481
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:
o 10 requests per second;
o limited to 60 requests per minute;
o 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:
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o the maximum number of allowed requests in the time window;
o the number of remaining requests in the current window;
o 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:
o each implementation associates different semantics to the same
header field names;
o 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. 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"}
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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 field 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-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:
o 5000 daily quota units;
o 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:
o it has only 100 quota units left;
o the window will reset in 10 hours.
Request:
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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"}
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 field 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-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:
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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}
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 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-Reset: 5
RateLimit-Limit: 100
Ratelimit-Remaining: 0
{
"title": "Too Many Requests",
"status": 429,
"detail": "You have exceeded your quota"
}
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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:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Policy: 100;w=60
Ratelimit-Remaining: 99
Ratelimit-Reset: 50
{"hello": "world"}
B.2.2. Dynamic limits with parameterized windows
The policy conveyed by the RateLimit-Limit field states that the
server accepts 100 quota units per minute.
To avoid resource exhaustion, the server artificially lowers the
actual limits returned in the throttling headers.
The RateLimit-Remaining field then advertises only 9 quota units for
the next 50 seconds to slow down the client.
Note that the server could have lowered even the other values in the
RateLimit-Limit 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:
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HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
RateLimit-Policy: 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.
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
RateLimit-Policy: 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
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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-Limit: 15
RateLimit-Policy: 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 field 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
field.
Request:
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GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"first": "request"}
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"second": "request"}
B.3.2. Use with multiple windows
This is a standardized way of describing the policy detailed in
Appendix B.1.2:
o 5000 daily quota units;
o 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:
o it has only 100 quota units left;
o the window will reset in 10 hours;
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o 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
RateLimit-Policy: 1000;w=3600, 5000;w=86400
RateLimit-Remaining: 100
RateLimit-Reset: 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.
2. Can I use RateLimit 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. 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
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only on [HTTP], in Section 1.1 we defined "scope" in terms of
URL.
Since clients are not required to process quota policies (see
Section 5), we could add a new "RateLimit-Scope" field to this
spec. See this discussion on a similar thread [2]
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 [3]
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 [4]
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 [5]
* 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?
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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.
8. Do a positive value of RateLimit-Remaining field imply any
service guarantee for my future requests to be served?
No. FAQ integrated in Section 3.3.
9. Is the quota-policy definition Section 2.1 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
RateLimit-Policy: 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 [6]
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.
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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, ..)

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 field and RateLimit-Reset
field for defining the throttling behavior.
RateLimit fields currently used on the web
This section is to be removed before publishing as an RFC.
Commonly used header field names are:
o "X-RateLimit-Limit", "X-RateLimit-Remaining", "X-RateLimit-Reset";
o "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:
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o "x-ratelimit-limit-minute", "x-ratelimit-limit-hour", "x-
ratelimit-limit-day"
o "x-ratelimit-remaining-minute", "x-ratelimit-remaining-hour", "x-
ratelimit-remaining-day"
Here are some interoperability issues:
o "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]
o 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 field value related to the ratio between the
current and the maximum throughput. e.g.
RateLimit-Limit: 12
RateLimit-Policy: 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
RateLimit-Policy: 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.
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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.
F.1. Since draft-ietf-httpapi-ratelimit-headers-03
This section is to be removed before publishing as an RFC.
o Split policy informatio in RateLimit-Policy #81
F.2. Since draft-ietf-httpapi-ratelimit-headers-02
This section is to be removed before publishing as an RFC.
o Address throttling scope #83
F.3. Since draft-ietf-httpapi-ratelimit-headers-01
This section is to be removed before publishing as an RFC.
o Update IANA considerations #60
o Use Structured fields #58
o Reorganize document #67
F.4. Since draft-ietf-httpapi-ratelimit-headers-00
This section is to be removed before publishing as an RFC.
o Use I-D.httpbis-semantics, which includes referencing delay-
seconds instead of delta-seconds. #5
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Authors' Addresses
Roberto Polli
Team Digitale, Italian Government
Italy
Email: robipolli@gmail.com
Alejandro Martinez Ruiz
Red Hat
Email: alex@flawedcode.org
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