Internet DRAFT - draft-ietf-httpapi-ratelimit-headers
draft-ietf-httpapi-ratelimit-headers
HTTPAPI R. Polli
Internet-Draft Team Digitale, Italian Government
Intended status: Standards Track A. Martinez
Expires: 26 December 2023 Red Hat
24 June 2023
RateLimit header fields for HTTP
draft-ietf-httpapi-ratelimit-headers-07
Abstract
This document defines the RateLimit-Policy and RateLimit HTTP header
fields for servers to advertise their service policy limits and the
current limits, thereby allowing clients to avoid being throttled.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-httpapi-ratelimit-
headers/.
Discussion of this document takes place on the HTTPAPI Working Group
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Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-httpapi/ratelimit-headers.
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This Internet-Draft will expire on 26 December 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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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. Service Limit . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Time Window . . . . . . . . . . . . . . . . . . . . . . . 6
3. RateLimit header field Definitions . . . . . . . . . . . . . 7
3.1. RateLimit . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Limit Keyword . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Remaining Keyword . . . . . . . . . . . . . . . . . . . . 8
3.4. Reset Keyword . . . . . . . . . . . . . . . . . . . . . . 8
3.5. RateLimit-Policy . . . . . . . . . . . . . . . . . . . . 9
4. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Performance Considerations . . . . . . . . . . . . . . . 10
5. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Intermediaries . . . . . . . . . . . . . . . . . . . . . 12
5.2. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.1. Throttling does not prevent clients from issuing
requests . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Information disclosure . . . . . . . . . . . . . . . . . 13
6.3. Remaining quota units are not granted requests . . . . . 13
6.4. Reliability of the reset keyword . . . . . . . . . . . . 13
6.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 14
6.5.1. Denial of Service . . . . . . . . . . . . . . . . . . 14
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8.1. RateLimit Keywords and Parameters Registration . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
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9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Rate-limiting and quotas . . . . . . . . . . . . . . 18
A.1. Interoperability issues . . . . . . . . . . . . . . . . . 19
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 20
B.1. Unparameterized responses . . . . . . . . . . . . . . . . 20
B.1.1. Throttling information in responses . . . . . . . . . 20
B.1.2. Use in conjunction with custom fields . . . . . . . . 21
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 25
B.3.1. Missing Remaining information . . . . . . . . . . . . 25
B.3.2. Use with multiple windows . . . . . . . . . . . . . . 26
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
RateLimit header fields currently used on the web . . . . . . . . 31
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 32
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Since draft-ietf-httpapi-ratelimit-headers-03 . . . . . . . . . 32
Since draft-ietf-httpapi-ratelimit-headers-02 . . . . . . . . . 32
Since draft-ietf-httpapi-ratelimit-headers-01 . . . . . . . . . 32
Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . . . . 33
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 header fields to enable rate
limiting:
* RateLimit: to convey the server's current limit of quota units
available to the client in the policy time window, the remaining
quota units in the current window, and the time remaining in the
current window, specified in seconds, and
* RateLimit-Policy: the service policy limits.
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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 header 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 header 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 header fields may be returned in
both successful (see Section 15.3 of [HTTP]) and non-successful
responses. This specification does not cover whether non
Successful responses count on quota usage, nor it mandates any
correlation between the RateLimit values and the returned status
code.
Throttling policy: This specification does not mandate a specific
throttling policy. The values published in the fields, including
the window size, can be statically or dynamically evaluated.
Service Level Agreement: Conveyed quota hints do not imply any
service guarantee. Server is free to throttle respectful clients
under certain circumstances.
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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 header fields defined in this document are collectively referred
to as "RateLimit header fields".
2. Concepts
2.1. Quota Policy
A quota policy is maintained by a server to limit the activity
(counted in quota units) of a given client over a period of time
(known as the time window (Section 2.3)) to a specified amount (known
as the service limit (Section 2.2)).
Quota policies can be advertised by servers (see Section 3.5), but
they are not required to be, and more than one quota policy can
affect a given request from a client to a server.
A quota policy is expressed in Structured Fields [STRUCTURED-FIELDS]
as an Integer that indicates the service limit with associated
parameters.
The following Parameters are defined in this specification:
w: The REQUIRED "w" parameter value conveys a time window
(Section 2.3).
For example, a quota policy of 100 quota units per minute is
expressed as:
100;w=60
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Other parameters are allowed and can be regarded as comments.
Parameters for use by more than one implementation or service ought
to be registered within the "Hypertext Transfer Protocol (HTTP)
RateLimit Parameters Registry", as described in Section 8.1.
Implementation- or service-specific parameters SHOULD be prefixed
parameters with a vendor identifier, e.g. acme-policy, acme-burst.
2.2. Service Limit
The service limit is a non-negative Integer indicating the maximum
amount of activity that the server is willing to accept from what it
identifies as the client (e.g., based upon originating IP or user
authentication) during a time window (Section 2.3).
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.
For example, a server could count requests like /books/{id} once, but
count search requests like /books?author=WuMing twice. This might
result in 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
Often, the service limit advertised will match the server's actual
limit. However, it MAY differ when weight mechanisms, bursts, or
other server policies are implemented. In that case the difference
SHOULD be communicated using an extension or documented separately.
2.3. Time Window
Quota 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.
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.
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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"
3. RateLimit header field Definitions
The following RateLimit response header fields are defined.
3.1. RateLimit
A server uses the "RateLimit" response header field to communicate
its quota policies.
The field is a Dictionary. The allowed keys are defined in the
"Hypertext Transfer Protocol (HTTP) RateLimit Keywords and Parameters
Registry", as described in Section 8.1.
The following Keys are defined in this specification:
limit: The REQUIRED "limit" key value conveys the expiring limit
(Section 3.2). remaining:
The OPTIONAL "remaining" key value conveys the remaining quota
units (Section 3.3). reset:
The REQUIRED "reset" key value conveys the time window reset time
(Section 3.4).
This specification does not define Parameters for this field. If
they appear, they MUST be ignored.
This field cannot appear in a trailer section.
3.2. Limit Keyword
The "limit" keyword indicates the service limit (Section 2.2)
associated with the client in the current time window (Section 2.3).
If the client exceeds that limit, it MAY not be served.
It is an Item and its value is a non-negative Integer referred to as
the "expiring limit". This specification does not define Parameters
for it. 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:
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* inferred by the value of the reset keyword (Section 3.4) at the
moment of the reset, or
* communicated out-of-band (e.g. in the documentation).
Example:
limit=100
The RateLimit-Policy header field (see Section 3.5), might contain
information on the associated time window.
3.3. Remaining Keyword
The "remaining" keyword indicates the remaining quota units
associated with the expiring-limit.
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 remaining value is a
guarantee that further requests will be served.
When the value of the remaining keyword is low, it indicates that the
server may soon throttle the client (see Section 4).
For example:
remaining=50
3.4. Reset Keyword
The "reset" keyword indicates the number of seconds until the
available quota units associated with the expiring-limit resets.
It is a non-negative Integer compatible with the delay-seconds rule,
because:
* it does not rely on clock synchronization and is resilient to
clock adjustment and clock skew between client and server (see
Section 5.6.7 of [HTTP]);
* it mitigates the risk related to thundering herd when too many
clients are serviced with the same timestamp.
This specification does not define Parameters for it. If they
appear, they MUST be ignored.
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For example:
reset=50
The client MUST NOT assume that all its service limit will be reset
at the moment indicated by the reset keyword. The server MAY
arbitrarily alter the reset keyword value between subsequent
requests; for example, in case of resource saturation or to implement
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
A server uses the RateLimit header fields to communicate its quota
policies. A response that includes the RateLimit-Limit header field
MUST also include the RateLimit-Reset. It MAY also include a
RateLimit-Remaining header field.
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A server MAY return RateLimit header fields independently of the
response status code. This includes on throttled responses. This
document does not mandate any correlation between the RateLimit
header field values and the returned status code.
Servers should be careful when returning RateLimit header fields in
redirection responses (i.e., responses with 3xx status codes) because
a low remaining keyword value could prevent the client from issuing
requests. For example, given the RateLimit header fields below, a
client could decide to wait 10 seconds before following the
"Location" header field (see Section 10.2.2 of [HTTP]), because the
remaining keyword value is 0.
HTTP/1.1 301 Moved Permanently
Location: /foo/123
RateLimit: limit=10, remaining=0, reset=10
If a response contains both the Retry-After and the RateLimit header
fields, the reset keyword value SHOULD reference the same point in
time as the Retry-After field value.
When using a policy involving more than one time window, the server
MUST reply with the RateLimit header fields related to the time
window with the lower remaining keyword values.
A service using RateLimit header fields MUST NOT convey values
exposing an unwanted volume of requests and SHOULD implement
mechanisms to cap the ratio between the remaining and the reset
keyword values (see Section 6.5); this is especially important when a
quota policy uses a large time window.
Under certain conditions, a server MAY artificially lower RateLimit
header field values between subsequent requests, e.g. to respond to
Denial of Service attacks or in case of resource saturation.
4.1. Performance Considerations
Servers are not required to return RateLimit header fields in every
response, and clients need to take this into account. For example,
an implementer concerned with performance might provide RateLimit
header fields only when a given quota is 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].
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5. Client Behavior
The RateLimit header fields can be used by clients to determine
whether the associated request respected the server's quota policy,
and as an indication of whether subsequent requests will. However,
the server might apply other criteria when servicing future requests,
and so the quota policy may not completely reflect whether they will
succeed.
For example, a successful response with the following fields:
RateLimit: limit=10, remaining=1, 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 2.2.
A client is responsible for ensuring that RateLimit header field
values returned cause reasonable client behavior with respect to
throughput and latency (see Section 6.5 and Section 6.5.1).
A client receiving RateLimit header fields MUST NOT assume that
future responses will contain the same RateLimit header fields, or
any RateLimit header fields at all.
Malformed RateLimit header fields MUST be ignored.
A client SHOULD NOT exceed the quota units conveyed by the remaining
keyword before the time window expressed in the reset keyword.
A client MAY still probe the server if the reset keyword is
considered too high.
The value of the reset keyword is generated at response time: a
client aware of a significant network latency MAY behave accordingly
and use other information (e.g. the "Date" response header field, or
otherwise gathered metrics) to better estimate the reset keyword
moment intended by the server.
The details provided in the RateLimit-Policy header field are
informative and MAY be ignored.
If a response contains both the RateLimit and Retry-After fields, the
Retry-After field MUST take precedence and the reset keyword MAY be
ignored.
This specification does not mandate a specific throttling behavior
and implementers can adopt their preferred policies, including:
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* slowing down or preemptively back-off their request rate when
approaching quota limits;
* consuming all the quota according to the exposed limits and then
wait.
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 header fields'
values in such a way as to communicate a more permissive quota
policy; this includes removing the RateLimit header fields.
An intermediary MAY alter the RateLimit header fields in such a way
as to communicate a more restrictive quota policy when:
* it is aware of the quota unit semantic used by the Origin Server;
* it implements this specification and enforces a quota policy which
is more restrictive than the one conveyed in the fields.
An intermediary SHOULD forward a request even when presuming that it
might not be serviced; the service returning the RateLimit header
fields is the sole responsible of enforcing the communicated quota
policy, and it is always free to service incoming requests.
This specification does not mandate any behavior on intermediaries
respect to retries, nor requires that intermediaries have any role in
respecting quota policies. For example, it is legitimate for a proxy
to retransmit a request without notifying the client, and thus
consuming quota units.
Privacy considerations (Section 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 header fields.
Because the information in RateLimit header fields on a cached
response may not be current, they SHOULD be ignored on responses that
come from cache (i.e., those with a positive current_age; see
Section 4.2.3 of [HTTP-CACHING]).
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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
client could probe the endpoint to get traffic information of another
user.
As intermediaries might retransmit requests and consume quota units
without prior knowledge of the user agent, RateLimit header fields
might reveal the existence of an intermediary to the user agent.
6.3. Remaining quota units are not granted requests
RateLimit header fields convey hints from the server to the clients
in order to help them avoid being throttled out.
Clients MUST NOT consider the quota units (Section 2.2) returned in
remaining keyword as a service level agreement.
In case of resource saturation, the server MAY artificially lower the
returned values or not serve the request regardless of the advertised
quotas.
6.4. Reliability of the reset keyword
Consider that service limit might not be restored after the moment
referenced by the reset keyword (Section 3.4), and the reset keyword
value may not be fixed nor constant.
Subsequent requests might return a higher reset keyword value to
limit concurrency or implement dynamic or adaptive throttling
policies.
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6.5. Resource exhaustion
When returning reset keyword 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 reset keyword accordingly
Date: Tue, 15 Nov 1994 08:00:00 GMT
RateLimit: limit=1, remaining=1, reset=36000
there's a high probability that all clients will show up at 18:00:00.
This could be mitigated by adding some jitter to the field-value.
Resource exhaustion issues can be associated with quota policies
using a large time window, because a user agent by chance or on
purpose might consume most of its quota units in a significantly
shorter interval.
This behavior can be even triggered by the provided RateLimit header
fields. The following example describes a service with an unconsumed
quota policy of 10000 quota units per 1000 seconds.
RateLimit: limit=10000, remaining=10000, reset=10
RateLimit-Policy: 10000;w=1000
A client implementing a simple ratio between remaining keyword and
reset keyword could infer an average throughput of 1000 quota units
per second, while the limit keyword conveys a quota-policy with an
average of 10 quota units per second. If the service cannot handle
such load, it should return either a lower remaining keyword value or
an higher reset keyword value. Moreover, complementing large time
window quota policies with a short time window one mitigates those
risks.
6.5.1. Denial of Service
RateLimit header fields may contain unexpected values by chance or on
purpose. For example, an excessively high remaining keyword value
may be:
* used by a malicious intermediary to trigger a Denial of Service
attack or consume client resources boosting its requests;
* passed by a misconfigured server;
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or a high reset keyword value could inhibit clients to contact the
server (e.g. similarly to receiving "Retry-after: 1000000").
To mitigate this risk, clients can set thresholds that they consider
reasonable in terms of quota units, time window, concurrent requests
or throughput, and define a consistent behavior when the RateLimit
exceed those thresholds. For example this means capping the maximum
number of request per second, or implementing retries when the reset
keyword exceeds ten minutes.
The considerations above are not limited to RateLimit header fields,
but apply to all fields affecting how clients behave in subsequent
requests (e.g. Retry-After).
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).
Since rate limiting is usually implemented in contexts where clients
are either identified or profiled (e.g. assigning different quota
units to different users), this is rarely a concern.
Privacy enhancing infrastructures using RateLimit header fields can
define specific techniques to mitigate the risks of re-
identification.
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 | permanent | Section 3.1 of RFC nnnn |
+------------------+-----------+-------------------------+
| RateLimit-Policy | permanent | Section 3.5 of RFC nnnn |
+------------------+-----------+-------------------------+
Table 1
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8.1. RateLimit Keywords and Parameters Registration
IANA is requested to create a new registry to be called "Hypertext
Transfer Protocol (HTTP) RateLimit Keywords and Parameters Registry",
to be located at https://www.iana.org/assignments/http-ratelimit-
parameters (https://www.iana.org/assignments/http-ratelimit-
parameters). Registration is done on the advice of a Designated
Expert, appointed by the IESG or their delegate. All entries are
Specification Required ([IANA], Section 4.6).
Registration requests consist of the following information:
* Token name: The keyword or parameter name, conforming to
[STRUCTURED-FIELDS].
* Token type: Whether the token is a Dictionary Keyword or a
Parameter Name.
* Field name: The RateLimit header field for which the parameter is
registered. If a parameter is intended to be used with multiple
fields, it has to be registered for each one.
* Description: A brief description of the parameter.
* Specification document: A reference to the document that specifies
the parameter, preferably including a URI that can be used to
retrieve a copy of the document.
* Comments (optional): Any additional information that can be
useful.
The initial contents of this registry should be:
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+==========+=========+==========+===========+=============+==========+
|Field Name|Token |Token type|Description|Specification|Comments |
| |name | | | |(optional)|
+==========+=========+==========+===========+=============+==========+
|RateLimit |limit |Dictionary|Expiring |Section 3.2 | |
| | |Key |limit |of RFC nnnn | |
+----------+---------+----------+-----------+-------------+----------+
|RateLimit |remaining|Dictionary|Remaining |Section 3.3 | |
| | |Key |quota units|of RFC nnnn | |
+----------+---------+----------+-----------+-------------+----------+
|RateLimit |reset |Dictionary|Quota reset|Section 3.4 | |
| | |Key |interval |of RFC nnnn | |
+----------+---------+----------+-----------+-------------+----------+
|RateLimit-|w |Parameter |Time window|Section 2.1 | |
|Policy | |name | |of RFC nnnn | |
+----------+---------+----------+-----------+-------------+----------+
Table 2
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,
<https://www.rfc-editor.org/rfc/rfc9110>.
[IANA] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.
[WEB-ORIGIN]
Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/rfc/rfc6454>.
9.2. Informative References
[HPACK] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/rfc/rfc7541>.
[HTTP-CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/rfc/rfc9111>.
[PRIVACY] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/rfc/rfc6973>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/rfc/rfc3339>.
[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/rfc/rfc6585>.
[UNIX] The Open Group, "The Single UNIX Specification, Version 2
- 6 Vol Set for UNIX 98", February 1997.
Appendix A. Rate-limiting and quotas
Servers use quota mechanisms to avoid systems overload, to ensure an
equitable distribution of computational resources or to enforce other
policies - e.g. monetization.
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A basic quota mechanism limits the number of acceptable requests in a
given time window, e.g. 10 requests per second.
When quota is exceeded, servers usually do not serve the request
replying instead with a 4xx HTTP status code (e.g. 429 or 403) or
adopt more aggressive policies like dropping connections.
Quotas may be enforced on different basis (e.g. per user, per IP, per
geographic area, ..) and at different levels. For example, an user
may be allowed to issue:
* 10 requests per second;
* limited to 60 requests per minute;
* limited to 1000 requests per hour.
Moreover system metrics, statistics and heuristics can be used to
implement more complex policies, where the number of acceptable
requests and the time window are computed dynamically.
To help clients throttling their requests, servers may expose the
counters used to evaluate quota policies via HTTP header fields.
Those response headers may be added by HTTP intermediaries such as
API gateways and reverse proxies.
On the web we can find many different rate-limit headers, usually
containing the number of allowed requests in a given time window, and
when the window is reset.
The common choice is to return three headers containing:
* the maximum number of allowed requests in the time window;
* the number of remaining requests in the current window;
* the time remaining in the current window expressed in seconds or
as a timestamp;
A.1. Interoperability issues
A major interoperability issue in throttling is the lack of standard
headers, because:
* each implementation associates different semantics to the same
header field names;
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* 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, remaining=0, reset=50
{"hello": "world"}
Since the field values are not necessarily correlated with the
response status code, a subsequent request is not required to fail.
The example below shows that the server decided to serve the request
even if remaining keyword value is 0. Another server, or the same
server under other load conditions, could have decided to throttle
the request instead.
Request:
GET /items/456 HTTP/1.1
Host: api.example
Response:
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HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: limit=100, remaining=0, reset=48
{"still": "successful"}
B.1.2. Use in conjunction with custom fields
The server uses two custom fields, namely acme-RateLimit-DayLimit and
acme-RateLimit-HourLimit to expose the following policy:
* 5000 daily quota units;
* 1000 hourly quota units.
The client consumed 4900 quota units in the first 14 hours.
Despite the next hourly limit of 1000 quota units, the closest limit
to reach is the daily one.
The server then exposes the RateLimit header fields to inform the
client that:
* it has only 100 quota units left;
* the window will reset in 10 hours.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
acme-RateLimit-DayLimit: 5000
acme-RateLimit-HourLimit: 1000
RateLimit: limit=5000, remaining=100, reset=36000
{"hello": "world"}
B.1.3. Use for limiting concurrency
RateLimit header fields may be used to limit concurrency, advertising
limits that are lower than the usual ones in case of saturation, thus
increasing availability.
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The server adopted a basic policy of 100 quota units per minute, and
in case of resource exhaustion adapts the returned values reducing
both limit and remaining keyword values.
After 2 seconds the client consumed 40 quota units
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: limit=100, remaining=60, reset=58
{"elapsed": 2, "issued": 40}
At the subsequent request - due to resource exhaustion - the server
advertises only remaining=20.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: limit=100, remaining=20, 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 header field
reference the same moment, but this is not a requirement.
The 429 (Too Many Request) HTTP status code is just used as an
example.
Request:
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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: limit=100, remaining=0, reset=5
{
"title": "Too Many Requests",
"status": 429,
"detail": "You have exceeded your quota"
}
B.2. Parameterized responses
B.2.1. Throttling window specified via parameter
The client has 99 quota units left for the next 50 seconds. The time
window is communicated by the w parameter, so we know the throughput
is 100 quota units per minute.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: limit=100, remaining=99, reset=50
RateLimit-Policy: 100;w=60
{"hello": "world"}
B.2.2. Dynamic limits with parameterized windows
The policy conveyed by the RateLimit header field states that the
server accepts 100 quota units per minute.
To avoid resource exhaustion, the server artificially lowers the
actual limits returned in the throttling headers.
The remaining keyword then advertises only 9 quota units for the next
50 seconds to slow down the client.
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Note that the server could have lowered even the other values in the
RateLimit header field: this specification does not mandate any
relation between the field values contained in subsequent responses.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: limit=10, remaining=9, reset=50
RateLimit-Policy: 100;w=60
{
"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 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, remaining=0, reset=20
RateLimit-Policy: 15;w=20
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
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B.3. Dynamic limits for pushing back with Retry-After and slow down
Alternatively, given the same context where the previous example
starts, we can convey the same information to the client via Retry-
After, with the advantage that the server can now specify the
policy's nominal limit and window that will apply after the reset,
e.g. assuming the resource exhaustion is likely to be gone by then,
so the advertised policy does not need to be adjusted, yet we managed
to stop requests for a while and slow down the rest of the current
window.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 429 Too Many Requests
Content-Type: application/json
Retry-After: 20
RateLimit: limit=15, remaining=15, reset=40
RateLimit-Policy: 100;w=60
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
Note that in this last response the client is expected to honor
Retry-After and perform no requests for the specified amount of time,
whereas the previous example would not force the client to stop
requests before the reset time is elapsed, as it would still be free
to query again the server even if it is likely to have the request
rejected.
B.3.1. Missing Remaining information
The server does not expose remaining keyword values (for example,
because the underlying counters are not available). Instead, it
resets the limit counter every second.
It communicates to the client the limit of 10 quota units per second
always returning the limit and reset keywords.
Request:
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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, 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, reset=1
{"second": "request"}
B.3.2. Use with multiple windows
This is a standardized way of describing the policy detailed in
Appendix B.1.2:
* 5000 daily quota units;
* 1000 hourly quota units.
The client consumed 4900 quota units in the first 14 hours.
Despite the next hourly limit of 1000 quota units, the closest limit
to reach is the daily one.
The server then exposes the RateLimit header fields to inform the
client that:
* it has only 100 quota units left;
* the window will reset in 10 hours;
* the expiring-limit is 5000.
Request:
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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=5000, remaining=100, reset=36000
RateLimit-Policy: 1000;w=3600, 5000;w=86400
{"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 header fields in throttled responses (eg with
status code 429)?
Yes, you can.
3. Are those specs tied to RFC 6585?
No. [RFC6585] defines the 429 status code and we use it just as
an example of a throttled request, that could instead use even
403 or whatever status code. The goal of this specification is
to standardize the name and semantic of three RateLimit header
fields widely used on the internet. Stricter relations with
status codes or error response payloads would impose behaviors to
all the existing implementations making the adoption more
complex.
4. Why don't pass the throttling scope as a parameter?
The word "scope" can have different meanings: for example it can
be an URL, or an authorization scope. Since authorization is out
of the scope of this document (see Section 1.1), and that we rely
only on [HTTP], in Section 1.1 we defined "scope" in terms of
URL.
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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
(https://github.com/httpwg/http-core/pull/317#issuecomment-
585868767)
Specific ecosystems can still bake their own prefixed parameters,
such as acme-auth-scope or acme-url-scope and ensure that clients
process them. This behavior cannot be relied upon when
communicating between different ecosystems.
We are open to suggestions: comment on this issue
(https://github.com/ioggstream/draft-polli-ratelimit-headers/
issues/70)
5. Why using delay-seconds instead of a UNIX Timestamp? Why not
using subsecond precision?
Using delay-seconds aligns with Retry-After, which is returned in
similar contexts, eg on 429 responses.
Timestamps require a clock synchronization protocol (see
Section 5.6.7 of [HTTP]). This may be problematic (e.g. clock
adjustment, clock skew, failure of hardcoded clock
synchronization servers, IoT devices, ..). Moreover timestamps
may not be monotonically increasing due to clock adjustment. See
Another NTP client failure story
(https://community.ntppool.org/t/another-ntp-client-failure-
story/1014/)
We did not use subsecond precision because:
* that is more subject to system clock correction like the one
implemented via the adjtimex() Linux system call;
* response-time latency may not make it worth. A brief
discussion on the subject is on the httpwg ml
(https://lists.w3.org/Archives/Public/ietf-http-
wg/2019JulSep/0202.html)
* almost all rate-limit headers implementations do not use it.
6. 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 the remaining and reset keywords.
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7. Shouldn't I limit concurrency instead of request rate?
You can use this specification to limit concurrency at the HTTP
level (see {#use-for-limiting-concurrency}) and help clients to
shape their requests avoiding being throttled out.
A problematic way to limit concurrency is connection dropping,
especially when connections are multiplexed (e.g. HTTP/2)
because this results in unserviced client requests, which is
something we want to avoid.
A semantic way to limit concurrency is to return 503 + Retry-
After in case of resource saturation (e.g. thrashing, connection
queues too long, Service Level Objectives not meet, ..).
Saturation conditions can be either dynamic or static: all this
is out of the scope for the current document.
8. Do a positive value of remaining keyword 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, remaining=50, 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, remaining=50, reset=44
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 (https://github.com/ioggstream/
draft-polli-ratelimit-headers/issues/34#issuecomment-519366481)
The basic form of those fields is easily parseable, even by
implementers processing responses using technologies like dynamic
interpreter with limited syntax.
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Using a single field complicates parsing and takes a significantly
different approach from the existing ones: this can limit adoption.
1. Why don't mention connections?
Beware of the term "connection":   - it is just _one_ possible
saturation cause. Once you go that path  you will expose other
infrastructural details (bandwidth, CPU, .. see Section 6.2) 
and complicate client compliance;  - it is an infrastructural
detail defined in terms of server and network  rather than the
consumed service. This specification protects the services
first, and then the infrastructures through client cooperation
(see Section 6.1).   RateLimit header 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 header fields?
Generally, they should not because it might result in unserviced
requests. There are reasonable use cases for intermediaries
mangling RateLimit header fields though, e.g. when they enforce
stricter quota-policies, or when they are an active component of
the service. In those case we will consider them as part of the
originating infrastructure.
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
the limit, remaining and reset keywords for defining the
throttling behavior.
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.
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RateLimit header fields currently used on the web
This section is to be removed before publishing as an RFC.
Commonly used header field names are:
* X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset;
* X-Rate-Limit-Limit, X-Rate-Limit-Remaining, X-Rate-Limit-Reset.
There are variants too, where the window is specified in the header
field name, eg:
* x-ratelimit-limit-minute, x-ratelimit-limit-hour, x-ratelimit-
limit-day
* x-ratelimit-remaining-minute, x-ratelimit-remaining-hour, x-
ratelimit-remaining-day
Here are some interoperability issues:
* X-RateLimit-Remaining references different values, depending on
the implementation:
- seconds remaining to the window expiration
- milliseconds remaining to the window expiration
- seconds since UTC, in UNIX Timestamp [UNIX]
- a datetime, either IMF-fixdate [HTTP] or [RFC3339]
* different headers, with the same semantic, are used by different
implementers:
- X-RateLimit-Limit and X-Rate-Limit-Limit
- X-RateLimit-Remaining and X-Rate-Limit-Remaining
- X-RateLimit-Reset and X-Rate-Limit-Reset
The semantic of RateLimit depends on the windowing algorithm. A
sliding window policy for example may result in having a remaining
keyword value related to the ratio between the current and the
maximum throughput. e.g.
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RateLimit: limit=12, \
remaining=6, \ ; using 50% of throughput, that is 6 units/s
reset=1
RateLimit-Policy: 12;w=1
If this is the case, the optimal solution is to achieve
RateLimit: limit=12, \
remaining=1 \ ; using 100% of throughput, that is 12 units/s
reset=1
RateLimit-Policy: 12;w=1
At this point you should stop increasing your request rate.
Acknowledgements
Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro
Ranellucci, Amos Jeffries, Martin Thomson, Erik Wilde and Mark
Nottingham for being the initial contributors of these
specifications. Kudos to the first community implementers: Aapo
Talvensaari, Nathan Friedly and Sanyam Dogra.
In addition to the people above, this document owes a lot to the
extensive discussion in the HTTPAPI workgroup, including Rich Salz,
Darrel Miller and Julian Reschke.
Changes
This section is to be removed before publishing as an RFC.
Since draft-ietf-httpapi-ratelimit-headers-03
This section is to be removed before publishing as an RFC.
* Split policy informatio in RateLimit-Policy #81
Since draft-ietf-httpapi-ratelimit-headers-02
This section is to be removed before publishing as an RFC.
* Address throttling scope #83
Since draft-ietf-httpapi-ratelimit-headers-01
This section is to be removed before publishing as an RFC.
* Update IANA considerations #60
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* Use Structured fields #58
* Reorganize document #67
Since draft-ietf-httpapi-ratelimit-headers-00
This section is to be removed before publishing as an RFC.
* Use I-D.httpbis-semantics, which includes referencing delay-
seconds instead of delta-seconds. #5
Authors' Addresses
Roberto Polli
Team Digitale, Italian Government
Italy
Email: robipolli@gmail.com
Alejandro Martinez Ruiz
Red Hat
Email: alex@flawedcode.org
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