draft-ietf-httpapi-ratelimit-headers-06.unpg.txt   draft-ietf-httpapi-ratelimit-headers-latest.txt 
HTTPAPI Working Group R. Polli HTTPAPI Working Group R. Polli
Internet-Draft Team Digitale, Italian Government Internet-Draft Team Digitale, Italian Government
Intended status: Standards Track A. Martinez Intended status: Standards Track A. Martinez
Expires: June 25, 2023 Red Hat Expires: July 12, 2024 Red Hat
December 22, 2022 January 09, 2024
RateLimit Fields for HTTP RateLimit header fields for HTTP
draft-ietf-httpapi-ratelimit-headers-06 draft-ietf-httpapi-ratelimit-headers-latest
Abstract Abstract
This document defines the RateLimit-Limit, RateLimit-Remaining, This document defines the RateLimit-Policy and RateLimit HTTP header
RateLimit-Reset and RateLimit-Policy HTTP fields for servers to fields for servers to advertise their service policy limits and the
advertise their current service rate limits, thereby allowing clients current limits, thereby allowing clients to avoid being throttled.
to avoid being throttled.
About This Document About This Document
This note is to be removed before publishing as an RFC. This note is to be removed before publishing as an RFC.
Status information for this document may be found at Status information for this document may be found at
<https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit- <https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit-
headers/>. headers/>.
Discussion of this document takes place on the HTTPAPI Working Group Discussion of this document takes place on the HTTPAPI Working Group
skipping to change at line 50 skipping to change at page 2, line 4
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Table of Contents Table of Contents
1. Introduction 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Goals 1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Notational Conventions 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Concepts 2. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Quota Policy 2.1. Quota Policy . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Time Window 2.2. Service Limit . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Service Limit 2.3. Time Window . . . . . . . . . . . . . . . . . . . . . . . 6
3. RateLimit Field Definitions 3. RateLimit header field Definitions . . . . . . . . . . . . . 7
3.1. RateLimit-Limit 3.1. RateLimit . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. RateLimit-Policy 3.2. Limit Keyword . . . . . . . . . . . . . . . . . . . . . . 7
3.3. RateLimit-Remaining 3.3. Remaining Keyword . . . . . . . . . . . . . . . . . . . . 8
3.4. RateLimit-Reset 3.4. Reset Keyword . . . . . . . . . . . . . . . . . . . . . . 8
4. Server Behavior 3.5. RateLimit-Policy . . . . . . . . . . . . . . . . . . . . 9
4.1. Performance Considerations 4. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
5. Client Behavior 4.1. Performance Considerations . . . . . . . . . . . . . . . 10
5.1. Intermediaries 5. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Caching 5.1. Intermediaries . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations 5.2. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Throttling does not prevent clients from issuing requests 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.2. Information disclosure 6.1. Throttling does not prevent clients from issuing requests 13
6.3. Remaining quota units are not granted requests 6.2. Information disclosure . . . . . . . . . . . . . . . . . 13
6.4. Reliability of RateLimit-Reset 6.3. Remaining quota units are not granted requests . . . . . 13
6.5. Resource exhaustion 6.4. Reliability of the reset keyword . . . . . . . . . . . . 13
6.5.1. Denial of Service 6.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 14
7. Privacy Considerations 6.5.1. Denial of Service . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
8.1. RateLimit Parameters Registration 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. References 8.1. RateLimit Keywords and Parameters Registration . . . . . 16
9.1. Normative References 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.2. Informative References 9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.3. URIs 9.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Rate-limiting and quotas 9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 18
A.1. Interoperability issues Appendix A. Rate-limiting and quotas . . . . . . . . . . . . . . 19
Appendix B. Examples A.1. Interoperability issues . . . . . . . . . . . . . . . . . 20
B.1. Unparameterized responses Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 20
B.1.1. Throttling information in responses B.1. Unparameterized responses . . . . . . . . . . . . . . . . 20
B.1.2. Use in conjunction with custom fields B.1.1. Throttling information in responses . . . . . . . . . 20
B.1.3. Use for limiting concurrency B.1.2. Use in conjunction with custom fields . . . . . . . . 21
B.1.4. Use in throttled responses B.1.3. Use for limiting concurrency . . . . . . . . . . . . 22
B.2. Parameterized responses B.1.4. Use in throttled responses . . . . . . . . . . . . . 23
B.2.1. Throttling window specified via parameter B.2. Parameterized responses . . . . . . . . . . . . . . . . . 23
B.2.2. Dynamic limits with parameterized windows B.2.1. Throttling window specified via parameter . . . . . . 24
B.2.3. Dynamic limits for pushing back and slowing down B.2.2. Dynamic limits with parameterized windows . . . . . . 24
B.2.3. Dynamic limits for pushing back and slowing down . . 25
B.3. Dynamic limits for pushing back with Retry-After and slow B.3. Dynamic limits for pushing back with Retry-After and slow
down down . . . . . . . . . . . . . . . . . . . . . . . . . . 25
B.3.1. Missing Remaining information B.3.1. Missing Remaining information . . . . . . . . . . . . 26
B.3.2. Use with multiple windows B.3.2. Use with multiple windows . . . . . . . . . . . . . . 27
FAQ FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
RateLimit fields currently used on the web RateLimit header fields currently used on the web . . . . . . . . 31
Acknowledgements Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 33
Changes Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
F.1. Since draft-ietf-httpapi-ratelimit-headers-03 F.1. Since draft-ietf-httpapi-ratelimit-headers-03 . . . . . . 33
F.2. Since draft-ietf-httpapi-ratelimit-headers-02 F.2. Since draft-ietf-httpapi-ratelimit-headers-02 . . . . . . 33
F.3. Since draft-ietf-httpapi-ratelimit-headers-01 F.3. Since draft-ietf-httpapi-ratelimit-headers-01 . . . . . . 33
F.4. Since draft-ietf-httpapi-ratelimit-headers-00 F.4. Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . 33
Authors' Addresses Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction 1. Introduction
Rate limiting HTTP clients has become a widespread practice, Rate limiting HTTP clients has become a widespread practice,
especially for HTTP APIs. Typically, servers who do so limit the especially for HTTP APIs. Typically, servers who do so limit the
number of acceptable requests in a given time window (e.g. 10 number of acceptable requests in a given time window (e.g. 10
requests per second). See Appendix A for further information on the requests per second). See Appendix A for further information on the
current usage of rate limiting in HTTP. current usage of rate limiting in HTTP.
Currently, there is no standard way for servers to communicate quotas Currently, there is no standard way for servers to communicate quotas
so that clients can throttle its requests to prevent errors. This so that clients can throttle its requests to prevent errors. This
document defines a set of standard HTTP fields to enable rate document defines a set of standard HTTP header fields to enable rate
limiting: limiting:
o RateLimit-Limit: the server's quota for requests by the client in o RateLimit: to convey the server's current limit of quota units
the time window, available to the client in the policy time window, the remaining
quota units in the current window, and the time remaining in the
o RateLimit-Remaining: the remaining quota in the current window, current window, specified in seconds, and
o RateLimit-Reset: the time remaining in the current window,
specified in seconds, and
o RateLimit-Policy: the quota policy. o RateLimit-Policy: the service policy limits.
These fields allow the establishment of complex rate limiting These fields allow the establishment of complex rate limiting
policies, including using multiple and variable time windows and policies, including using multiple and variable time windows and
dynamic quotas, and implementing concurrency limits. dynamic quotas, and implementing concurrency limits.
The behavior of the RateLimit-Reset field is compatible with the The behavior of the RateLimit header field is compatible with the
delay-seconds notation of Retry-After. delay-seconds notation of Retry-After.
1.1. Goals 1.1. Goals
The goals of this document are: The goals of this document are:
Interoperability: Standardization of the names and semantics of Interoperability: Standardization of the names and semantics of
rate-limit headers to ease their enforcement and adoption; rate-limit headers to ease their enforcement and adoption;
Resiliency: Improve resiliency of HTTP infrastructure by providing Resiliency: Improve resiliency of HTTP infrastructure by providing
clients with information useful to throttle their requests and clients with information useful to throttle their requests and
prevent 4xx or 5xx responses; prevent 4xx or 5xx responses;
Documentation: Simplify API documentation by eliminating the need to Documentation: Simplify API documentation by eliminating the need to
include detailed quota limits and related fields in API include detailed quota limits and related fields in API
documentation. documentation.
The following features are out of the scope of this document: The following features are out of the scope of this document:
Authorization: RateLimit fields are not meant to support Authorization: RateLimit header fields are not meant to support
authorization or other kinds of access controls. authorization or other kinds of access controls.
Throttling scope: This specification does not cover the throttling Throttling scope: This specification does not cover the throttling
scope, that may be the given resource-target, its parent path or scope, that may be the given resource-target, its parent path or
the whole Origin (see Section 7 of [WEB-ORIGIN]). This can be the whole Origin (see Section 7 of [WEB-ORIGIN]). This can be
addressed using extensibility mechanisms such as the parameter addressed using extensibility mechanisms such as the parameter
registry Section 8.1. registry Section 8.1.
Response status code: RateLimit fields may be returned in both Response status code: RateLimit header fields may be returned in
successful (see Section 15.3 of [HTTP]) and non-successful both successful (see Section 15.3 of [HTTP]) and non-successful
responses. This specification does not cover whether non responses. This specification does not cover whether non
Successful responses count on quota usage, nor it mandates any Successful responses count on quota usage, nor it mandates any
correlation between the RateLimit values and the returned status correlation between the RateLimit values and the returned status
code. code.
Throttling policy: This specification does not mandate a specific Throttling policy: This specification does not mandate a specific
throttling policy. The values published in the fields, including throttling policy. The values published in the fields, including
the window size, can be statically or dynamically evaluated. the window size, can be statically or dynamically evaluated.
Service Level Agreement: Conveyed quota hints do not imply any Service Level Agreement: Conveyed quota hints do not imply any
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by [RFC7405] along with the "#rule" extension defined in by [RFC7405] along with the "#rule" extension defined in
Section 5.6.1 of [HTTP]. Section 5.6.1 of [HTTP].
The term Origin is to be interpreted as described in Section 7 of The term Origin is to be interpreted as described in Section 7 of
[WEB-ORIGIN]. [WEB-ORIGIN].
This document uses the terms List, Item and Integer from Section 3 of This document uses the terms List, Item and Integer from Section 3 of
[STRUCTURED-FIELDS] to specify syntax and parsing, along with the [STRUCTURED-FIELDS] to specify syntax and parsing, along with the
concept of "bare item". concept of "bare item".
The fields defined in this document are collectively referred to as The header fields defined in this document are collectively referred
"RateLimit fields". to as "RateLimit header fields".
2. Concepts 2. Concepts
2.1. Quota Policy 2.1. Quota Policy
A quota policy is described in terms of quota units (Section 2.3) and A quota policy is maintained by a server to limit the activity
a time window (Section 2.2). It is an Item whose bare item is a (counted in quota units) of a given client over a period of time
service limit (Section 2.3), along with associated Parameters. (known as the time window (Section 2.3)) to a specified amount (known
as the service limit (Section 2.2)).
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 Quota policies can be advertised by servers (see Section 3.5), but
ought to be registered within the "Hypertext Transfer Protocol (HTTP) they are not required to be, and more than one quota policy can
RateLimit Parameters Registry", as described in Section 8.1. affect a given request from a client to a server.
For example, a quota policy of 100 quota units per minute: A quota policy is expressed in Structured Fields [STRUCTURED-FIELDS]
as an Integer that indicates the service limit with associated
parameters.
100;w=60 The following Parameters are defined in this specification:
The definition of a quota policy does not imply any specific w: The REQUIRED "w" parameter value conveys a time window
distribution of quota units within the time window. If applicable, (Section 2.3).
these details can be conveyed as extension parameters.
For example, two quota policies containing further details via For example, a quota policy of 100 quota units per minute is
extension parameters: expressed as:
100;w=60;comment="fixed window" 100;w=60
12;w=1;burst=1000;policy="leaky bucket"
To avoid clashes, implementers SHOULD prefix unregistered parameters Other parameters are allowed and can be regarded as comments.
with a vendor identifier, e.g. "acme-policy", "acme-burst". While it Parameters for use by more than one implementation or service ought
is useful to define a clear syntax and semantics even for custom to be registered within the "Hypertext Transfer Protocol (HTTP)
parameters, it is important to note that user agents are not required RateLimit Parameters Registry", as described in Section 8.1.
to process quota policy information.
2.2. Time Window Implementation- or service-specific parameters SHOULD be prefixed
parameters with a vendor identifier, e.g. "acme-policy", "acme-
burst".
Rate limit policies limit the number of acceptable requests within a 2.2. Service Limit
given time interval, known as a time window.
The time window is a non-negative Integer value expressing that The service limit is a non-negative Integer indicating the maximum
interval in seconds, similar to the "delay-seconds" rule defined in amount of activity that the server is willing to accept from what it
Section 10.2.3 of [HTTP]. Subsecond precision is not supported. identifies as the client (e.g., based upon originating IP or user
authentication) during a time window (Section 2.3).
2.3. Service Limit The activity being limited is usually the HTTP requests made by the
client; for example "you can make 100 requests per minute". However,
a server might only rate limit some requests (based upon URI, method,
user identity, etc.), and it might weigh requests differently.
Therefore, quota policies are defined in terms of "quota units".
Servers SHOULD document how they count quota units.
The service limit is associated with the maximum number of requests For example, a server could count requests like "/books/{id}" once,
that the server is willing to accept from one or more clients on a but count search requests like "/books?author=WuMing" twice. This
given basis (originating IP, authenticated user, geographical, ..) might result in the following counters:
during a time window (Section 2.2).
The service limit is a non-negative Integer expressed in quota units. 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
The service limit SHOULD match the maximum number of acceptable Often, the service limit advertised will match the server's actual
requests. However, the service limit MAY differ from the total limit. However, it MAY differ when weight mechanisms, bursts, or
number of acceptable requests when weight mechanisms, bursts, or other server policies are implemented. In that case the difference
other server policies are implemented. SHOULD be communicated using an extension or documented separately.
If the service limit does not match the maximum number of acceptable 2.3. Time Window
requests the relation with that SHOULD be communicated out-of-band.
Example: A server could Quota policies limit the number of acceptable requests within a given
time interval, known as a time window.
o count once requests like "/books/{id}" 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.
o count twice search requests like "/books?author=WuMing" By default, a quota policy does not constrain the distribution of
quota units within the time window. If necessary, these details can
be conveyed as extension parameters.
so that we have the following counters For example, two quota policies containing further details via
extension parameters:
GET /books/123 ; service-limit=4, remaining: 3, status=200 100;w=60;comment="fixed window"
GET /books?author=WuMing ; service-limit=4, remaining: 1, status=200 12;w=1;burst=1000;policy="leaky bucket"
GET /books?author=Eco ; service-limit=4, remaining: 0, status=429
3. RateLimit Field Definitions 3. RateLimit header field Definitions
The following RateLimit response fields are defined. The following RateLimit response header fields are defined.
3.1. RateLimit-Limit 3.1. RateLimit
The "RateLimit-Limit" response field indicates the service limit A server uses the "RateLimit" response header field to communicate
(Section 2.3) associated with the client in the current time window its quota policies.
(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 The field is a Dictionary. The allowed keys are defined in the
to as the "expiring-limit". This specification does not define "Hypertext Transfer Protocol (HTTP) RateLimit Keywords and Parameters
Parameters for this field. If they appear, they MUST be ignored. Registry", as described in Section 8.1.
The expiring-limit MUST be set to the service limit that is closest The following Keys are defined in this specification:
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 limit: The REQUIRED "limit" key value conveys the expiring limit
the reset, or (Section 3.2). remaining:
o communicated out-of-band (e.g. in the documentation). The OPTIONAL "remaining" key value conveys the remaining quota
units (Section 3.3). reset:
The RateLimit-Policy field (see Section 3.2), might contain The REQUIRED "reset" key value conveys the time window reset time
information on the associated time window. (Section 3.4).
RateLimit-Limit: 100 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. This field cannot appear in a trailer section.
3.2. RateLimit-Policy 3.2. Limit Keyword
The "RateLimit-Policy" response field indicates the quota policies The "limit" keyword indicates the service limit (Section 2.2)
currently associated with the client. Its value is informative. associated with the client in the current time window (Section 2.3).
If the client exceeds that limit, it MAY not be served.
The field is a non-empty List of Items. Each item is a quota policy It is an Item and its value is a non-negative Integer referred to as
(Section 2.1). the "expiring limit". This specification does not define Parameters
for it. If they appear, they MUST be ignored.
This field can convey the time window associated with the expiring- The expiring limit MUST be set to the service limit that is closest
limit, as shown in this example: to reaching its limit, and the associated time window MUST either be:
RateLimit-Policy: 100;w=10 o inferred by the value of the reset keyword (Section 3.4) at the
RateLimit-Limit: 100 moment of the reset, or
These examples show multiple policies being returned: o communicated out-of-band (e.g. in the documentation).
RateLimit-Policy: 10;w=1, 50;w=60, 1000;w=3600, 5000;w=86400 Example:
RateLimit-Policy: 10;w=1;burst=1000, 1000;w=3600
This field can be sent in a trailer section. limit=100
3.3. RateLimit-Remaining The RateLimit-Policy header field (see Section 3.5), might contain
information on the associated time window.
The "RateLimit-Remaining" response field indicates the remaining 3.3. Remaining Keyword
quota units associated to the expiring-limit.
The field is an Item and its value is a non-negative Integer The "remaining" keyword indicates the remaining quota units
expressed in quota units (Section 2.3). This specification does not associated with the expiring-limit.
define Parameters for this field. If they appear, they MUST be
ignored.
This field can be sent in a trailer section. It is an Item and its value is a non-negative Integer expressed in
quota units (Section 2.2). This specification does not define
Parameters for it. If they appear, they MUST be ignored.
Clients MUST NOT assume that a positive RateLimit-Remaining field Clients MUST NOT assume that a positive remaining value is a
value is a guarantee that further requests will be served. guarantee that further requests will be served.
When the value of RateLimit-Remaining is low, it indicates that the When the value of the remaining keyword is low, it indicates that the
server may soon throttle the client (see Section 4). server may soon throttle the client (see Section 4).
For example: For example:
RateLimit-Remaining: 50 remaining=50
3.4. RateLimit-Reset 3.4. Reset Keyword
The "RateLimit-Reset" field response field indicates the number of The "reset" keyword indicates the number of seconds until the
seconds until the quota associated to the expiring-limit resets. available quota units associated with the expiring-limit resets.
The field is a non-negative Integer compatible with the delay-seconds It is a non-negative Integer compatible with the delay-seconds rule,
rule, because: because:
o it does not rely on clock synchronization and is resilient to o it does not rely on clock synchronization and is resilient to
clock adjustment and clock skew between client and server (see clock adjustment and clock skew between client and server (see
Section 5.6.7 of [HTTP]); Section 5.6.7 of [HTTP]);
o it mitigates the risk related to thundering herd when too many o it mitigates the risk related to thundering herd when too many
clients are serviced with the same timestamp. clients are serviced with the same timestamp.
This specification does not define Parameters for this field. If This specification does not define Parameters for it. If they
they appear, they MUST be ignored. appear, they MUST be ignored.
This field can be sent in a trailer section.
An example of RateLimit-Reset field use is below. For example:
RateLimit-Reset: 50 reset=50
The client MUST NOT assume that all its service limit will be reset 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 at the moment indicated by the reset keyword. The server MAY
arbitrarily alter the RateLimit-Reset field value between subsequent arbitrarily alter the reset keyword value between subsequent
requests; for example, in case of resource saturation or to implement requests; for example, in case of resource saturation or to implement
sliding window policies. sliding window policies.
3.5. RateLimit-Policy
The "RateLimit-Policy" response header field indicates a service
policy 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). Two quota policies MUST NOT be associated with the
same quota units value.
This field can convey the time window associated with the expiring-
limit, as shown in this example:
RateLimit-Policy: 100;w=10
RateLimit: limit=100, remaining=50, reset=5
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
An example of invalid header field value with two policies associated
with the same quota units:
RateLimit-Policy: 10;w=1, 10;w=60
This field cannot appear in a trailer section.
4. Server Behavior 4. Server Behavior
A server uses the RateLimit fields to communicate its quota policies. A server uses the RateLimit header fields to communicate its quota
Sending the RateLimit-Limit and RateLimit-Reset fields is REQUIRED; policies. A response that includes the RateLimit-Limit header field
sending RateLimit-Remaining field is RECOMMENDED. MUST also include the RateLimit-Reset. It MAY also include a
RateLimit-Remaining header field.
A server MAY return RateLimit fields independently of the response A server MAY return RateLimit header fields independently of the
status code. This includes on throttled responses. This document response status code. This includes on throttled responses. This
does not mandate any correlation between the RateLimit field values document does not mandate any correlation between the RateLimit
and the returned status code. header field values and the returned status code.
Servers should be careful when returning RateLimit fields in Servers should be careful when returning RateLimit header fields in
redirection responses (i.e., responses with 3xx status codes) because redirection responses (i.e., responses with 3xx status codes) because
a low RateLimit-Remaining field value could prevent the client from a low remaining keyword value could prevent the client from issuing
issuing requests. For example, given the RateLimit fields below, a requests. For example, given the RateLimit header fields below, a
client could decide to wait 10 seconds before following the client could decide to wait 10 seconds before following the
"Location" header field (see Section 10.2.2 of [HTTP]), because the "Location" header field (see Section 10.2.2 of [HTTP]), because the
RateLimit-Remaining field value is 0. remaining keyword value is 0.
HTTP/1.1 301 Moved Permanently HTTP/1.1 301 Moved Permanently
Location: /foo/123 Location: /foo/123
RateLimit-Remaining: 0 RateLimit: limit=10, remaining=0, reset=10
RateLimit-Limit: 10
RateLimit-Reset: 10
If a response contains both the Retry-After and the RateLimit-Reset If a response contains both the Retry-After and the RateLimit header
fields, the RateLimit-Reset field value SHOULD reference the same fields, the reset keyword value SHOULD reference the same point in
point in time as the Retry-After field value. time as the Retry-After field value.
When using a policy involving more than one time window, the server When using a policy involving more than one time window, the server
MUST reply with the RateLimit fields related to the time window with MUST reply with the RateLimit header fields related to the time
the lower RateLimit-Remaining field values. window with the lower remaining keyword values.
A service using RateLimit fields MUST NOT convey values exposing an A service using RateLimit header fields MUST NOT convey values
unwanted volume of requests and SHOULD implement mechanisms to cap exposing an unwanted volume of requests and SHOULD implement
the ratio between RateLimit-Remaining and RateLimit-Reset field mechanisms to cap the ratio between the remaining and the reset
values (see Section 6.5); this is especially important when a quota keyword values (see Section 6.5); this is especially important when a
policy uses a large time window. quota policy uses a large time window.
Under certain conditions, a server MAY artificially lower RateLimit Under certain conditions, a server MAY artificially lower RateLimit
field values between subsequent requests, e.g. to respond to Denial header field values between subsequent requests, e.g. to respond to
of Service attacks or in case of resource saturation. 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 4.1. Performance Considerations
Servers are not required to return RateLimit fields in every Servers are not required to return RateLimit header fields in every
response, and clients need to take this into account. For example, response, and clients need to take this into account. For example,
an implementer concerned with performance might provide RateLimit an implementer concerned with performance might provide RateLimit
fields only when a given quota is going to expire. header fields only when a given quota is going to expire.
Implementers concerned with response fields' size, might take into Implementers concerned with response fields' size, might take into
account their ratio with respect to the content length, or use account their ratio with respect to the content length, or use
header-compression HTTP features such as [HPACK]. header-compression HTTP features such as [HPACK].
5. Client Behavior 5. Client Behavior
The RateLimit fields can be used by clients to determine whether the The RateLimit header fields can be used by clients to determine
associated request respected the server's quota policy, and as an whether the associated request respected the server's quota policy,
indication of whether subsequent requests will. However, the server and as an indication of whether subsequent requests will. However,
might apply other criteria when servicing future requests, and so the the server might apply other criteria when servicing future requests,
quota policy may not completely reflect whether they will succeed. and so the quota policy may not completely reflect whether they will
succeed.
For example, a successful response with the following fields: For example, a successful response with the following fields:
RateLimit-Limit: 10 RateLimit: limit=10, remaining=1, reset=7
RateLimit-Remaining: 1
RateLimit-Reset: 7
does not guarantee that the next request will be successful. does not guarantee that the next request will be successful.
Servers' behavior may be subject to other conditions like the one Servers' behavior may be subject to other conditions like the one
shown in the example from Section 2.3. shown in the example from Section 2.2.
A client MUST validate the RateLimit fields before using them and A client is responsible for ensuring that RateLimit header field
check if there are significant discrepancies with the expected ones. values returned cause reasonable client behavior with respect to
This includes a RateLimit-Reset field moment too far in the future throughput and latency (see Section 6.5 and Section 6.5.1).
(e.g. similarly to receiving "Retry-after: 1000000") or a service-
limit too high.
A client receiving RateLimit fields MUST NOT assume that future A client receiving RateLimit header fields MUST NOT assume that
responses will contain the same RateLimit fields, or any RateLimit future responses will contain the same RateLimit header fields, or
fields at all. any RateLimit header fields at all.
Malformed RateLimit fields MUST be ignored. Malformed RateLimit header fields MUST be ignored.
A client SHOULD NOT exceed the quota units conveyed by the RateLimit- A client SHOULD NOT exceed the quota units conveyed by the remaining
Remaining field before the time window expressed in RateLimit-Reset keyword before the time window expressed in the reset keyword.
field.
A client MAY still probe the server if the RateLimit-Reset field is A client MAY still probe the server if the reset keyword is
considered too high. considered too high.
The value of RateLimit-Reset field is generated at response time: a The value of the reset keyword is generated at response time: a
client aware of a significant network latency MAY behave accordingly client aware of a significant network latency MAY behave accordingly
and use other information (e.g. the "Date" response header field, or and use other information (e.g. the "Date" response header field, or
otherwise gathered metrics) to better estimate the RateLimit-Reset otherwise gathered metrics) to better estimate the reset keyword
field moment intended by the server. moment intended by the server.
The details provided in RateLimit-Policy field are informative and The details provided in the RateLimit-Policy header field are
MAY be ignored. informative and MAY be ignored.
If a response contains both the RateLimit-Reset and Retry-After If a response contains both the RateLimit and Retry-After fields, the
fields, the Retry-After field MUST take precedence and the RateLimit- Retry-After field MUST take precedence and the reset keyword MAY be
Reset field MAY be ignored. ignored.
This specification does not mandate a specific throttling behavior This specification does not mandate a specific throttling behavior
and implementers can adopt their preferred policies, including: and implementers can adopt their preferred policies, including:
o slowing down or preemptively back-off their request rate when o slowing down or preemptively back-off their request rate when
approaching quota limits; approaching quota limits;
o consuming all the quota according to the exposed limits and then o consuming all the quota according to the exposed limits and then
wait. wait.
5.1. Intermediaries 5.1. Intermediaries
This section documents the considerations advised in Section 16.3.2 This section documents the considerations advised in Section 16.3.2
of [HTTP]. of [HTTP].
An intermediary that is not part of the originating service An intermediary that is not part of the originating service
infrastructure and is not aware of the quota policy semantic used by infrastructure and is not aware of the quota policy semantic used by
the Origin Server SHOULD NOT alter the RateLimit fields' values in the Origin Server SHOULD NOT alter the RateLimit header fields'
such a way as to communicate a more permissive quota policy; this values in such a way as to communicate a more permissive quota
includes removing the RateLimit fields. policy; this includes removing the RateLimit header fields.
An intermediary MAY alter the RateLimit fields in such a way as to An intermediary MAY alter the RateLimit header fields in such a way
communicate a more restrictive quota policy when: 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 is aware of the quota unit semantic used by the Origin Server;
o it implements this specification and enforces a quota policy which o it implements this specification and enforces a quota policy which
is more restrictive than the one conveyed in the fields. is more restrictive than the one conveyed in the fields.
An intermediary SHOULD forward a request even when presuming that it An intermediary SHOULD forward a request even when presuming that it
might not be serviced; the service returning the RateLimit fields is might not be serviced; the service returning the RateLimit header
the sole responsible of enforcing the communicated quota policy, and fields is the sole responsible of enforcing the communicated quota
it is always free to service incoming requests. policy, and it is always free to service incoming requests.
This specification does not mandate any behavior on intermediaries This specification does not mandate any behavior on intermediaries
respect to retries, nor requires that intermediaries have any role in respect to retries, nor requires that intermediaries have any role in
respecting quota policies. For example, it is legitimate for a proxy respecting quota policies. For example, it is legitimate for a proxy
to retransmit a request without notifying the client, and thus to retransmit a request without notifying the client, and thus
consuming quota units. consuming quota units.
Privacy considerations (Section 7) provide further guidance on Privacy considerations (Section 7) provide further guidance on
intermediaries. intermediaries.
5.2. Caching 5.2. Caching
[HTTP-CACHING] defines how responses can be stored and reused for [HTTP-CACHING] defines how responses can be stored and reused for
subsequent requests, including those with RateLimit fields. Because subsequent requests, including those with RateLimit header fields.
the information in RateLimit fields on a cached response may not be Because the information in RateLimit header fields on a cached
current, they SHOULD be ignored on responses that come from cache response may not be current, they SHOULD be ignored on responses that
(i.e., those with a positive current_age; see Section 4.2.3 of come from cache (i.e., those with a positive current_age; see
[HTTP-CACHING]). Section 4.2.3 of [HTTP-CACHING]).
6. Security Considerations 6. Security Considerations
6.1. Throttling does not prevent clients from issuing requests 6.1. Throttling does not prevent clients from issuing requests
This specification does not prevent clients from making requests. This specification does not prevent clients from making requests.
Servers should always implement mechanisms to prevent resource Servers should always implement mechanisms to prevent resource
exhaustion. exhaustion.
6.2. Information disclosure 6.2. Information disclosure
skipping to change at line 579 skipping to change at page 13, line 26
information that can be used to saturate its infrastructural information that can be used to saturate its infrastructural
resources. resources.
While this specification does not mandate whether non-successful While this specification does not mandate whether non-successful
responses consume quota, if error responses (such as 401 responses consume quota, if error responses (such as 401
(Unauthorized) and 403 (Forbidden)) count against quota, a malicious (Unauthorized) and 403 (Forbidden)) count against quota, a malicious
client could probe the endpoint to get traffic information of another client could probe the endpoint to get traffic information of another
user. user.
As intermediaries might retransmit requests and consume quota units As intermediaries might retransmit requests and consume quota units
without prior knowledge of the user agent, RateLimit fields might without prior knowledge of the user agent, RateLimit header fields
reveal the existence of an intermediary to the user agent. might reveal the existence of an intermediary to the user agent.
6.3. Remaining quota units are not granted requests 6.3. Remaining quota units are not granted requests
RateLimit fields convey hints from the server to the clients in order RateLimit header fields convey hints from the server to the clients
to help them avoid being throttled out. in order to help them avoid being throttled out.
Clients MUST NOT consider the quota units (Section 2.3) returned in Clients MUST NOT consider the quota units (Section 2.2) returned in
RateLimit-Remaining field as a service level agreement. remaining keyword as a service level agreement.
In case of resource saturation, the server MAY artificially lower the In case of resource saturation, the server MAY artificially lower the
returned values or not serve the request regardless of the advertised returned values or not serve the request regardless of the advertised
quotas. quotas.
6.4. Reliability of RateLimit-Reset 6.4. Reliability of the reset keyword
Consider that service limit might not be restored after the moment Consider that service limit might not be restored after the moment
referenced by RateLimit-Reset field, and the RateLimit-Reset field referenced by the reset keyword (Section 3.4), and the reset keyword
value may not be fixed nor constant. value may not be fixed nor constant.
Subsequent requests might return a higher RateLimit-Reset field value Subsequent requests might return a higher reset keyword value to
to limit concurrency or implement dynamic or adaptive throttling limit concurrency or implement dynamic or adaptive throttling
policies. policies.
6.5. Resource exhaustion 6.5. Resource exhaustion
When returning RateLimit-Reset field you must be aware that many When returning reset keyword you must be aware that many throttled
throttled clients may come back at the very moment specified. clients may come back at the very moment specified.
This is true for Retry-After too. This is true for Retry-After too.
For example, if the quota resets every day at "18:00:00" and your For example, if the quota resets every day at "18:00:00" and your
server returns the RateLimit-Reset field accordingly server returns the reset keyword accordingly
Date: Tue, 15 Nov 1994 08:00:00 GMT Date: Tue, 15 Nov 1994 08:00:00 GMT
RateLimit-Reset: 36000 RateLimit: limit=1, remaining=1, reset=36000
there's a high probability that all clients will show up at there's a high probability that all clients will show up at
"18:00:00". "18:00:00".
This could be mitigated by adding some jitter to the field-value. This could be mitigated by adding some jitter to the field-value.
Resource exhaustion issues can be associated with quota policies Resource exhaustion issues can be associated with quota policies
using a large time window, because a user agent by chance or on using a large time window, because a user agent by chance or on
purpose might consume most of its quota units in a significantly purpose might consume most of its quota units in a significantly
shorter interval. shorter interval.
This behavior can be even triggered by the provided RateLimit fields. This behavior can be even triggered by the provided RateLimit header
The following example describes a service with an unconsumed quota fields. The following example describes a service with an unconsumed
policy of 10000 quota units per 1000 seconds. quota policy of 10000 quota units per 1000 seconds.
RateLimit-Limit: 10000 RateLimit: limit=10000, remaining=10000, reset=10
RateLimit-Policy: 10000;w=1000 RateLimit-Policy: 10000;w=1000
RateLimit-Remaining: 10000
RateLimit-Reset: 10
A client implementing a simple ratio between RateLimit-Remaining A client implementing a simple ratio between remaining keyword and
field and RateLimit-Reset field could infer an average throughput of reset keyword could infer an average throughput of 1000 quota units
1000 quota units per second, while the RateLimit-Limit field conveys per second, while the limit keyword conveys a quota-policy with an
a quota-policy with an average of 10 quota units per second. If the average of 10 quota units per second. If the service cannot handle
service cannot handle such load, it should return either a lower such load, it should return either a lower remaining keyword value or
RateLimit-Remaining field value or an higher RateLimit-Reset field an higher reset keyword value. Moreover, complementing large time
value. Moreover, complementing large time window quota policies with window quota policies with a short time window one mitigates those
a short time window one mitigates those risks. risks.
6.5.1. Denial of Service 6.5.1. Denial of Service
RateLimit fields may contain unexpected values by chance or on RateLimit header fields may contain unexpected values by chance or on
purpose. For example, an excessively high RateLimit-Remaining field purpose. For example, an excessively high remaining keyword value
value may be: may be:
o used by a malicious intermediary to trigger a Denial of Service o used by a malicious intermediary to trigger a Denial of Service
attack or consume client resources boosting its requests; attack or consume client resources boosting its requests;
o passed by a misconfigured server; o passed by a misconfigured server;
or a high RateLimit-Reset field value could inhibit clients to or a high reset keyword value could inhibit clients to contact the
contact the server. server (e.g. similarly to receiving "Retry-after: 1000000").
Clients MUST validate the received values to mitigate those risks. To mitigate this risk, clients can set thresholds that they consider
reasonable in terms of quota units, time window, concurrent requests
or throughput, and define a consistent behavior when the RateLimit
exceed those thresholds. For example this means capping the maximum
number of request per second, or implementing retries when the reset
keyword exceeds ten minutes.
The considerations above are not limited to RateLimit header fields,
but apply to all fields affecting how clients behave in subsequent
requests (e.g. Retry-After).
7. Privacy Considerations 7. Privacy Considerations
Clients that act upon a request to rate limit are potentially re- Clients that act upon a request to rate limit are potentially re-
identifiable (see Section 5.2.1 of [PRIVACY]) because they react to identifiable (see Section 5.2.1 of [PRIVACY]) because they react to
information that might only be given to them. Note that this might information that might only be given to them. Note that this might
apply to other fields too (e.g. Retry-After). apply to other fields too (e.g. Retry-After).
Since rate limiting is usually implemented in contexts where clients Since rate limiting is usually implemented in contexts where clients
are either identified or profiled (e.g. assigning different quota are either identified or profiled (e.g. assigning different quota
units to different users), this is rarely a concern. units to different users), this is rarely a concern.
Privacy enhancing infrastructures using RateLimit fields can define Privacy enhancing infrastructures using RateLimit header fields can
specific techniques to mitigate the risks of re-identification. define specific techniques to mitigate the risks of re-
identification.
8. IANA Considerations 8. IANA Considerations
IANA is requested to update one registry and create one new registry. IANA is requested to update one registry and create one new registry.
Please add the following entries to the "Hypertext Transfer Protocol Please add the following entries to the "Hypertext Transfer Protocol
(HTTP) Field Name Registry" registry ([HTTP]): (HTTP) Field Name Registry" registry ([HTTP]):
+---------------------+-----------+-------------------------+ +------------------+-----------+-------------------------+
| Field Name | Status | Specification | | Field Name | Status | Specification |
+---------------------+-----------+-------------------------+ +------------------+-----------+-------------------------+
| RateLimit-Limit | permanent | Section 3.1 of RFC nnnn | | RateLimit | permanent | Section 3.1 of RFC nnnn |
| RateLimit-Remaining | permanent | Section 3.3 of RFC nnnn | | RateLimit-Policy | permanent | Section 3.5 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 8.1. RateLimit Keywords and Parameters Registration
IANA is requested to create a new registry to be called "Hypertext IANA is requested to create a new registry to be called "Hypertext
Transfer Protocol (HTTP) RateLimit Parameters Registry", to be Transfer Protocol (HTTP) RateLimit Keywords and Parameters Registry",
located at https://www.iana.org/assignments/http-ratelimit-parameters to be located at https://www.iana.org/assignments/http-ratelimit-
[1]. Registration is done on the advice of a Designated Expert, parameters [1]. Registration is done on the advice of a Designated
appointed by the IESG or their delegate. All entries are Expert, appointed by the IESG or their delegate. All entries are
Specification Required ([IANA], Section 4.6). Specification Required ([IANA], Section 4.6).
Registration requests consist of the following information: Registration requests consist of the following information:
o Parameter name: The parameter name, conforming to o Token name: The keyword or parameter name, conforming to
[STRUCTURED-FIELDS]. [STRUCTURED-FIELDS].
o Field name: The RateLimit field for which the parameter is o Token type: Whether the token is a Dictionary Keyword or a
Parameter Name.
o Field name: The RateLimit header field for which the parameter is
registered. If a parameter is intended to be used with multiple registered. If a parameter is intended to be used with multiple
fields, it has to be registered for each one. fields, it has to be registered for each one.
o Description: A brief description of the parameter. o Description: A brief description of the parameter.
o Specification document: A reference to the document that specifies o Specification document: A reference to the document that specifies
the parameter, preferably including a URI that can be used to the parameter, preferably including a URI that can be used to
retrieve a copy of the document. retrieve a copy of the document.
o Comments (optional): Any additional information that can be o Comments (optional): Any additional information that can be
useful. useful.
The initial contents of this registry should be: The initial contents of this registry should be:
+----------------+----------+------------+--------------+-----------+ +-------------+--------+---------+----------+------------+----------+
| Field Name | Paramete | Descriptio | Specificatio | Comments | | Field Name | Token | Token | Descript | Specificat | Comments |
| | r name | n | n | (optional | | | name | type | ion | ion | (optiona |
| | | | | ) | | | | | | | l) |
+----------------+----------+------------+--------------+-----------+ +-------------+--------+---------+----------+------------+----------+
| RateLimit- | w | Time | Section 2.1 | | | RateLimit | limit | Diction | Expiring | Section 3. | |
| Policy | | window | of RFC nnnn | | | | | ary Key | limit | 2 of RFC | |
+----------------+----------+------------+--------------+-----------+ | | | | | nnnn | |
| RateLimit | remain | Diction | Remainin | Section 3. | |
| | ing | ary Key | g quota | 3 of RFC | |
| | | | units | nnnn | |
| RateLimit | reset | Diction | Quota | Section 3. | |
| | | ary Key | reset | 4 of RFC | |
| | | | interval | nnnn | |
| RateLimit- | w | Paramet | Time | Section 2. | |
| Policy | | er name | window | 1 of RFC | |
| | | | | nnnn | |
+-------------+--------+---------+----------+------------+----------+
9. References 9. References
9.1. Normative References 9.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110, Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022, DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>. <https://www.rfc-editor.org/info/rfc9110>.
skipping to change at line 762 skipping to change at page 18, line 10
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014, RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>. <https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[STRUCTURED-FIELDS] [STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>. <https://www.rfc-editor.org/info/rfc8941>.
[WEB-ORIGIN] [WEB-ORIGIN]
Barth, A., "The Web Origin Concept", RFC 6454, Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011, DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>. <https://www.rfc-editor.org/info/rfc6454>.
9.2. Informative References 9.2. Informative References
skipping to change at line 899 skipping to change at page 21, line 7
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 100 RateLimit: limit=100, remaining=0, reset=50
Ratelimit-Remaining: 0
Ratelimit-Reset: 50
{"hello": "world"} {"hello": "world"}
Since the field values are not necessarily correlated with the Since the field values are not necessarily correlated with the
response status code, a subsequent request is not required to fail. response status code, a subsequent request is not required to fail.
The example below shows that the server decided to serve the request The example below shows that the server decided to serve the request
even if RateLimit-Remaining field value is 0. Another server, or the even if remaining keyword value is 0. Another server, or the same
same server under other load conditions, could have decided to server under other load conditions, could have decided to throttle
throttle the request instead. the request instead.
Request: Request:
GET /items/456 HTTP/1.1 GET /items/456 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 100 RateLimit: limit=100, remaining=0, reset=48
Ratelimit-Remaining: 0
Ratelimit-Reset: 48
{"still": "successful"} {"still": "successful"}
B.1.2. Use in conjunction with custom fields B.1.2. Use in conjunction with custom fields
The server uses two custom fields, namely "acme-RateLimit-DayLimit" The server uses two custom fields, namely "acme-RateLimit-DayLimit"
and "acme-RateLimit-HourLimit" to expose the following policy: and "acme-RateLimit-HourLimit" to expose the following policy:
o 5000 daily quota units; o 5000 daily quota units;
o 1000 hourly quota units. o 1000 hourly quota units.
The client consumed 4900 quota units in the first 14 hours. The client consumed 4900 quota units in the first 14 hours.
Despite the next hourly limit of 1000 quota units, the closest limit Despite the next hourly limit of 1000 quota units, the closest limit
to reach is the daily one. to reach is the daily one.
The server then exposes the RateLimit fields to inform the client The server then exposes the RateLimit header fields to inform the
that: client that:
o it has only 100 quota units left; o it has only 100 quota units left;
o the window will reset in 10 hours. o the window will reset in 10 hours.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
acme-RateLimit-DayLimit: 5000 acme-RateLimit-DayLimit: 5000
acme-RateLimit-HourLimit: 1000 acme-RateLimit-HourLimit: 1000
RateLimit-Limit: 5000 RateLimit: limit=5000, remaining=100, reset=36000
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"} {"hello": "world"}
B.1.3. Use for limiting concurrency B.1.3. Use for limiting concurrency
Throttling fields may be used to limit concurrency, advertising RateLimit header fields may be used to limit concurrency, advertising
limits that are lower than the usual ones in case of saturation, thus limits that are lower than the usual ones in case of saturation, thus
increasing availability. increasing availability.
The server adopted a basic policy of 100 quota units per minute, and The server adopted a basic policy of 100 quota units per minute, and
in case of resource exhaustion adapts the returned values reducing in case of resource exhaustion adapts the returned values reducing
both RateLimit-Limit and RateLimit-Remaining field values. both limit and remaining keyword values.
After 2 seconds the client consumed 40 quota units After 2 seconds the client consumed 40 quota units
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 100 RateLimit: limit=100, remaining=60, reset=58
RateLimit-Remaining: 60
RateLimit-Reset: 58
{"elapsed": 2, "issued": 40} {"elapsed": 2, "issued": 40}
At the subsequent request - due to resource exhaustion - the server At the subsequent request - due to resource exhaustion - the server
advertises only "RateLimit-Remaining: 20". advertises only "remaining=20".
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 100 RateLimit: limit=100, remaining=20, reset=56
RateLimit-Remaining: 20
RateLimit-Reset: 56
{"elapsed": 4, "issued": 41} {"elapsed": 4, "issued": 41}
B.1.4. Use in throttled responses B.1.4. Use in throttled responses
A client exhausted its quota and the server throttles it sending A client exhausted its quota and the server throttles it sending
Retry-After. Retry-After.
In this example, the values of Retry-After and RateLimit-Reset field In this example, the values of Retry-After and RateLimit header field
reference the same moment, but this is not a requirement. reference the same moment, but this is not a requirement.
The 429 (Too Many Request) HTTP status code is just used as an The 429 (Too Many Request) HTTP status code is just used as an
example. example.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 429 Too Many Requests HTTP/1.1 429 Too Many Requests
Content-Type: application/json Content-Type: application/json
Date: Mon, 05 Aug 2019 09:27:00 GMT Date: Mon, 05 Aug 2019 09:27:00 GMT
Retry-After: Mon, 05 Aug 2019 09:27:05 GMT Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
RateLimit-Reset: 5 RateLimit: limit=100, remaining=0, reset=5
RateLimit-Limit: 100
Ratelimit-Remaining: 0
{ {
"title": "Too Many Requests", "title": "Too Many Requests",
"status": 429, "status": 429,
"detail": "You have exceeded your quota" "detail": "You have exceeded your quota"
} }
B.2. Parameterized responses B.2. Parameterized responses
B.2.1. Throttling window specified via parameter B.2.1. Throttling window specified via parameter
The client has 99 quota units left for the next 50 seconds. The time 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 window is communicated by the "w" parameter, so we know the
throughput is 100 quota units per minute. throughput is 100 quota units per minute.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 100 RateLimit: limit=100, remaining=99, reset=50
RateLimit-Policy: 100;w=60 RateLimit-Policy: 100;w=60
Ratelimit-Remaining: 99
Ratelimit-Reset: 50
{"hello": "world"} {"hello": "world"}
B.2.2. Dynamic limits with parameterized windows B.2.2. Dynamic limits with parameterized windows
The policy conveyed by the RateLimit-Limit field states that the The policy conveyed by the RateLimit header field states that the
server accepts 100 quota units per minute. server accepts 100 quota units per minute.
To avoid resource exhaustion, the server artificially lowers the To avoid resource exhaustion, the server artificially lowers the
actual limits returned in the throttling headers. actual limits returned in the throttling headers.
The RateLimit-Remaining field then advertises only 9 quota units for The remaining keyword then advertises only 9 quota units for the next
the next 50 seconds to slow down the client. 50 seconds to slow down the client.
Note that the server could have lowered even the other values in the Note that the server could have lowered even the other values in the
RateLimit-Limit field: this specification does not mandate any RateLimit header field: this specification does not mandate any
relation between the field values contained in subsequent responses. relation between the field values contained in subsequent responses.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 10 RateLimit: limit=10, remaining=9, reset=50
RateLimit-Policy: 100;w=60 RateLimit-Policy: 100;w=60
Ratelimit-Remaining: 9
Ratelimit-Reset: 50
{ {
"status": 200, "status": 200,
"detail": "Just slow down without waiting." "detail": "Just slow down without waiting."
} }
B.2.3. Dynamic limits for pushing back and slowing down B.2.3. Dynamic limits for pushing back and slowing down
Continuing the previous example, let's say the client waits 10 Continuing the previous example, let's say the client waits 10
seconds and performs a new request which, due to resource exhaustion, seconds and performs a new request which, due to resource exhaustion,
the server rejects and pushes back, advertising "RateLimit-Remaining: the server rejects and pushes back, advertising "remaining=0" for the
0" for the next 20 seconds. next 20 seconds.
The server advertises a smaller window with a lower limit to slow 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 down the client for the rest of its original window after the 20
seconds elapse. seconds elapse.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 429 Too Many Requests HTTP/1.1 429 Too Many Requests
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 0 RateLimit: limit=0, remaining=0, reset=20
RateLimit-Policy: 15;w=20 RateLimit-Policy: 15;w=20
Ratelimit-Remaining: 0
Ratelimit-Reset: 20
{ {
"status": 429, "status": 429,
"detail": "Wait 20 seconds, then slow down!" "detail": "Wait 20 seconds, then slow down!"
} }
B.3. Dynamic limits for pushing back with Retry-After and slow down B.3. Dynamic limits for pushing back with Retry-After and slow down
Alternatively, given the same context where the previous example Alternatively, given the same context where the previous example
starts, we can convey the same information to the client via Retry- starts, we can convey the same information to the client via Retry-
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Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 429 Too Many Requests HTTP/1.1 429 Too Many Requests
Content-Type: application/json Content-Type: application/json
Retry-After: 20 Retry-After: 20
RateLimit-Limit: 15 RateLimit: limit=15, remaining=15, reset=40
RateLimit-Policy: 100;w=60 RateLimit-Policy: 100;w=60
Ratelimit-Remaining: 15
Ratelimit-Reset: 40
{ {
"status": 429, "status": 429,
"detail": "Wait 20 seconds, then slow down!" "detail": "Wait 20 seconds, then slow down!"
} }
Note that in this last response the client is expected to honor Note that in this last response the client is expected to honor
Retry-After and perform no requests for the specified amount of time, Retry-After and perform no requests for the specified amount of time,
whereas the previous example would not force the client to stop whereas the previous example would not force the client to stop
requests before the reset time is elapsed, as it would still be free 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 to query again the server even if it is likely to have the request
rejected. rejected.
B.3.1. Missing Remaining information B.3.1. Missing Remaining information
The server does not expose RateLimit-Remaining field values (for The server does not expose remaining keyword values (for example,
example, because the underlying counters are not available). because the underlying counters are not available). Instead, it
Instead, it resets the limit counter every second. resets the limit counter every second.
It communicates to the client the limit of 10 quota units per second It communicates to the client the limit of 10 quota units per second
always returning the couple RateLimit-Limit and RateLimit-Reset always returning the limit and reset keywords.
field.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 10 RateLimit: limit=10, reset=1
Ratelimit-Reset: 1
{"first": "request"} {"first": "request"}
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 Ok HTTP/1.1 200 Ok
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 10 RateLimit: limit=10, reset=1
Ratelimit-Reset: 1
{"second": "request"} {"second": "request"}
B.3.2. Use with multiple windows B.3.2. Use with multiple windows
This is a standardized way of describing the policy detailed in This is a standardized way of describing the policy detailed in
Appendix B.1.2: Appendix B.1.2:
o 5000 daily quota units; o 5000 daily quota units;
o 1000 hourly quota units. o 1000 hourly quota units.
The client consumed 4900 quota units in the first 14 hours. The client consumed 4900 quota units in the first 14 hours.
Despite the next hourly limit of 1000 quota units, the closest limit Despite the next hourly limit of 1000 quota units, the closest limit
to reach is the daily one. to reach is the daily one.
The server then exposes the RateLimit fields to inform the client The server then exposes the RateLimit header fields to inform the
that: client that:
o it has only 100 quota units left; o it has only 100 quota units left;
o the window will reset in 10 hours; o the window will reset in 10 hours;
o the expiring-limit is 5000. o the expiring-limit is 5000.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
skipping to change at line 1233 skipping to change at page 28, line 4
o it has only 100 quota units left; o it has only 100 quota units left;
o the window will reset in 10 hours; o the window will reset in 10 hours;
o the expiring-limit is 5000. o the expiring-limit is 5000.
Request: Request:
GET /items/123 HTTP/1.1 GET /items/123 HTTP/1.1
Host: api.example Host: api.example
Response: Response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/json Content-Type: application/json
RateLimit-Limit: 5000 RateLimit: limit=5000, remaining=100, reset=36000
RateLimit-Policy: 1000;w=3600, 5000;w=86400 RateLimit-Policy: 1000;w=3600, 5000;w=86400
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"} {"hello": "world"}
FAQ FAQ
This section is to be removed before publishing as an RFC. This section is to be removed before publishing as an RFC.
1. Why defining standard fields for throttling? 1. Why defining standard fields for throttling?
To simplify enforcement of throttling policies. To simplify enforcement of throttling policies.
2. Can I use RateLimit fields in throttled responses (eg with status 2. Can I use RateLimit header fields in throttled responses (eg with
code 429)? status code 429)?
Yes, you can. Yes, you can.
3. Are those specs tied to RFC 6585? 3. Are those specs tied to RFC 6585?
No. [RFC6585] defines the "429" status code and we use it just No. [RFC6585] defines the "429" status code and we use it just
as an example of a throttled request, that could instead use even as an example of a throttled request, that could instead use even
"403" or whatever status code. The goal of this specification is "403" or whatever status code. The goal of this specification is
to standardize the name and semantic of three ratelimit fields to standardize the name and semantic of three RateLimit header
widely used on the internet. Stricter relations with status fields widely used on the internet. Stricter relations with
codes or error response payloads would impose behaviors to all status codes or error response payloads would impose behaviors to
the existing implementations making the adoption more complex. all the existing implementations making the adoption more
complex.
4. Why don't pass the throttling scope as a parameter? 4. Why don't pass the throttling scope as a parameter?
The word "scope" can have different meanings: for example it can The word "scope" can have different meanings: for example it can
be an URL, or an authorization scope. Since authorization is out 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 of the scope of this document (see Section 1.1), and that we rely
only on [HTTP], in Section 1.1 we defined "scope" in terms of only on [HTTP], in Section 1.1 we defined "scope" in terms of
URL. URL.
Since clients are not required to process quota policies (see Since clients are not required to process quota policies (see
skipping to change at line 1315 skipping to change at page 29, line 37
* response-time latency may not make it worth. A brief * response-time latency may not make it worth. A brief
discussion on the subject is on the httpwg ml [5] discussion on the subject is on the httpwg ml [5]
* almost all rate-limit headers implementations do not use it. * almost all rate-limit headers implementations do not use it.
6. Why not support multiple quota remaining? 6. Why not support multiple quota remaining?
While this might be of some value, my experience suggests that While this might be of some value, my experience suggests that
overly-complex quota implementations results in lower overly-complex quota implementations results in lower
effectiveness of this policy. This spec allows the client to effectiveness of this policy. This spec allows the client to
easily focusing on RateLimit-Remaining and RateLimit-Reset. easily focusing on the remaining and reset keywords.
7. Shouldn't I limit concurrency instead of request rate? 7. Shouldn't I limit concurrency instead of request rate?
You can use this specification to limit concurrency at the HTTP You can use this specification to limit concurrency at the HTTP
level (see {#use-for-limiting-concurrency}) and help clients to level (see {#use-for-limiting-concurrency}) and help clients to
shape their requests avoiding being throttled out. shape their requests avoiding being throttled out.
A problematic way to limit concurrency is connection dropping, A problematic way to limit concurrency is connection dropping,
especially when connections are multiplexed (e.g. HTTP/2) especially when connections are multiplexed (e.g. HTTP/2)
because this results in unserviced client requests, which is because this results in unserviced client requests, which is
something we want to avoid. something we want to avoid.
A semantic way to limit concurrency is to return 503 + Retry- A semantic way to limit concurrency is to return 503 + Retry-
After in case of resource saturation (e.g. thrashing, connection After in case of resource saturation (e.g. thrashing, connection
queues too long, Service Level Objectives not meet, ..). queues too long, Service Level Objectives not meet, ..).
Saturation conditions can be either dynamic or static: all this Saturation conditions can be either dynamic or static: all this
is out of the scope for the current document. is out of the scope for the current document.
8. Do a positive value of RateLimit-Remaining field imply any 8. Do a positive value of remaining keyword imply any service
service guarantee for my future requests to be served? guarantee for my future requests to be served?
No. FAQ integrated in Section 3.3. No. FAQ integrated in Section 3.3.
9. Is the quota-policy definition Section 2.1 too complex? 9. Is the quota-policy definition Section 2.1 too complex?
You can always return the simplest form of the 3 fields You can always return the simplest form of the 3 fields
RateLimit-Limit: 100 RateLimit: limit=100, remaining=50, reset=60
RateLimit-Remaining: 50
RateLimit-Reset: 60
The key runtime value is the first element of the list: "expiring- The key runtime value is the first element of the list: "expiring-
limit", the others quota-policy are informative. So for the limit", the others quota-policy are informative. So for the
following field: following field:
RateLimit-Limit: 100 RateLimit: limit=100, remaining=50, reset=44
RateLimit-Policy: 100;w=60;burst=1000;comment="sliding window", 5000;w=3600;burst=0;comment="fixed window" 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" 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? 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 The most common syntax we found on the web is "X-RateLimit-*" and
when starting this I-D we opted for it [6] when starting this I-D we opted for it [6]
The basic form of those fields is easily parseable, even by The basic form of those fields is easily parseable, even by
skipping to change at line 1378 skipping to change at page 30, line 50
1. Why don't mention connections? 1. Why don't mention connections?
Beware of the term "connection": &#65532; &#65532; - it is just Beware of the term "connection": &#65532; &#65532; - it is just
_one_ possible saturation cause. Once you go that path &#65532; _one_ possible saturation cause. Once you go that path &#65532;
you will expose other infrastructural details (bandwidth, CPU, .. you will expose other infrastructural details (bandwidth, CPU, ..
see Section 6.2) &#65532; and complicate client compliance; see Section 6.2) &#65532; and complicate client compliance;
&#65532; - it is an infrastructural detail defined in terms of &#65532; - it is an infrastructural detail defined in terms of
server and network &#65532; rather than the consumed service. server and network &#65532; rather than the consumed service.
This specification protects the services first, and then the This specification protects the services first, and then the
infrastructures through client cooperation (see Section 6.1). infrastructures through client cooperation (see Section 6.1).
&#65532; &#65532; RateLimit fields enable sending _on the same &#65532; &#65532; RateLimit header fields enable sending _on the
connection_ different limit values &#65532; on each response, same connection_ different limit values &#65532; on each
depending on the policy scope (e.g. per-user, per-custom-key, ..) response, depending on the policy scope (e.g. per-user, per-
&#65532; custom-key, ..) &#65532;
2. Can intermediaries alter RateLimit fields? 2. Can intermediaries alter RateLimit header fields?
Generally, they should not because it might result in unserviced Generally, they should not because it might result in unserviced
requests. There are reasonable use cases for intermediaries requests. There are reasonable use cases for intermediaries
mangling RateLimit fields though, e.g. when they enforce stricter mangling RateLimit header fields though, e.g. when they enforce
quota-policies, or when they are an active component of the stricter quota-policies, or when they are an active component of
service. In those case we will consider them as part of the the service. In those case we will consider them as part of the
originating infrastructure. originating infrastructure.
3. Why the "w" parameter is just informative? Could it be used by a 3. Why the "w" parameter is just informative? Could it be used by a
client to determine the request rate? client to determine the request rate?
A non-informative "w" parameter might be fine in an environment A non-informative "w" parameter might be fine in an environment
where clients and servers are tightly coupled. Conveying where clients and servers are tightly coupled. Conveying
policies with this detail on a large scale would be very complex policies with this detail on a large scale would be very complex
and implementations would be likely not interoperable. We thus and implementations would be likely not interoperable. We thus
decided to leave "w" as an informational parameter and only rely decided to leave "w" as an informational parameter and only rely
on RateLimit-Limit, RateLimit-Remaining field and RateLimit-Reset on the limit, remaining and reset keywords for defining the
field for defining the throttling behavior. throttling behavior.
RateLimit fields currently used on the web 4. Can I use RateLimit fields in trailers? Servers usually
establish whether the request is in-quota before creating a
response, so the RateLimit field values should be already
available in that moment. Supporting trailers has the only
advantage that allows to provide more up-to-date information to
the client in case of slow responses. However, this complicates
client implementations with respect to combining fields from
headers and accounting for intermediaries that drop trailers.
Since there are no current implementations that use trailers, we
decided to leave this as a future-work.
RateLimit header fields currently used on the web
This section is to be removed before publishing as an RFC. This section is to be removed before publishing as an RFC.
Commonly used header field names are: Commonly used header field names are:
o "X-RateLimit-Limit", "X-RateLimit-Remaining", "X-RateLimit-Reset"; o "X-RateLimit-Limit", "X-RateLimit-Remaining", "X-RateLimit-Reset";
o "X-Rate-Limit-Limit", "X-Rate-Limit-Remaining", "X-Rate-Limit- o "X-Rate-Limit-Limit", "X-Rate-Limit-Remaining", "X-Rate-Limit-
Reset". Reset".
skipping to change at line 1445 skipping to change at page 32, line 33
o different headers, with the same semantic, are used by different o different headers, with the same semantic, are used by different
implementers: implementers:
* X-RateLimit-Limit and X-Rate-Limit-Limit * X-RateLimit-Limit and X-Rate-Limit-Limit
* X-RateLimit-Remaining and X-Rate-Limit-Remaining * X-RateLimit-Remaining and X-Rate-Limit-Remaining
* X-RateLimit-Reset and X-Rate-Limit-Reset * X-RateLimit-Reset and X-Rate-Limit-Reset
The semantic of RateLimit-Remaining depends on the windowing The semantic of RateLimit depends on the windowing algorithm. A
algorithm. A sliding window policy for example may result in having sliding window policy for example may result in having a remaining
a RateLimit-Remaining field value related to the ratio between the keyword value related to the ratio between the current and the
current and the maximum throughput. e.g. maximum throughput. e.g.
RateLimit-Limit: 12 RateLimit: limit=12, \
RateLimit-Policy: 12;w=1 remaining=6, \ ; using 50% of throughput, that is 6 units/s
RateLimit-Remaining: 6 ; using 50% of throughput, that is 6 units/s reset=1
RateLimit-Reset: 1 RateLimit-Policy: 12;w=1
If this is the case, the optimal solution is to achieve If this is the case, the optimal solution is to achieve
RateLimit-Limit: 12 RateLimit: limit=12, \
remaining=1 \ ; using 100% of throughput, that is 12 units/s
reset=1
RateLimit-Policy: 12;w=1 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. At this point you should stop increasing your request rate.
Acknowledgements Acknowledgements
Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro
Ranellucci, Amos Jeffries, Martin Thomson, Erik Wilde and Mark Ranellucci, Amos Jeffries, Martin Thomson, Erik Wilde and Mark
Nottingham for being the initial contributors of these Nottingham for being the initial contributors of these
specifications. Kudos to the first community implementers: Aapo specifications. Kudos to the first community implementers: Aapo
Talvensaari, Nathan Friedly and Sanyam Dogra. Talvensaari, Nathan Friedly and Sanyam Dogra.
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