RateLimit header fields for HTTP
draft-ietf-httpapi-ratelimit-headers-08
| Document | Type | Active Internet-Draft (httpapi WG) | |
|---|---|---|---|
| Authors | Roberto Polli , Alex Martínez Ruiz , Darrel Miller | ||
| Last updated | 2024-10-07 | ||
| Replaces | draft-polli-ratelimit-headers | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-httpapi-ratelimit-headers-08
HTTPAPI R. Polli
Internet-Draft Team Digitale, Italian Government
Intended status: Standards Track A. Martinez
Expires: 10 April 2025 Red Hat
D. Miller
Microsoft
7 October 2024
RateLimit header fields for HTTP
draft-ietf-httpapi-ratelimit-headers-08
Abstract
This document defines the RateLimit-Policy and RateLimit HTTP header
fields for servers to advertise their quota policies and the current
service 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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information can be found at https://datatracker.ietf.org/wg/httpapi/
about/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-httpapi/ratelimit-headers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. RateLimit-Policy Field . . . . . . . . . . . . . . . . . . . 6
3.1. Quota Policy Item . . . . . . . . . . . . . . . . . . . . 6
3.1.1. Quota Parameter . . . . . . . . . . . . . . . . . . . 6
3.1.2. Quota Unit Parameter . . . . . . . . . . . . . . . . 6
3.1.3. Window Parameter . . . . . . . . . . . . . . . . . . 7
3.1.4. Partition Key Parameter . . . . . . . . . . . . . . . 7
3.2. RateLimit Policy Field Examples . . . . . . . . . . . . . 7
4. RateLimit Field . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Service Limit Item . . . . . . . . . . . . . . . . . . . 8
4.1.1. Remaining Parameter . . . . . . . . . . . . . . . . . 8
4.1.2. Reset Parameter . . . . . . . . . . . . . . . . . . . 8
4.1.3. Partition Key Parameter . . . . . . . . . . . . . . . 9
4.2. RateLimit Field Examples . . . . . . . . . . . . . . . . 9
5. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Generating Partition Keys . . . . . . . . . . . . . . . . 10
5.2. Performance Considerations . . . . . . . . . . . . . . . 10
6. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Consuming Partition Keys . . . . . . . . . . . . . . . . 12
6.2. Intermediaries . . . . . . . . . . . . . . . . . . . . . 12
6.3. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
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7.1. Throttling does not prevent clients from issuing
requests . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2. Information disclosure . . . . . . . . . . . . . . . . . 13
7.3. Remaining quota units are not granted requests . . . . . 14
7.4. Reliability of the reset parameter . . . . . . . . . . . 14
7.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 14
7.5.1. Denial of Service . . . . . . . . . . . . . . . . . . 15
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9.1. RateLimit quota unit registry . . . . . . . . . . . . . . 16
9.1.1. Registration Template . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Rate-limiting and quotas . . . . . . . . . . . . . . 18
A.1. Interoperability issues . . . . . . . . . . . . . . . . . 19
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 19
B.1. Responses without defining policies . . . . . . . . . . . 19
B.1.1. Throttling information in responses . . . . . . . . . 20
B.1.2. Multiple policies in response . . . . . . . . . . . . 20
B.1.3. Use for limiting concurrency . . . . . . . . . . . . 21
B.1.4. Use in throttled responses . . . . . . . . . . . . . 22
B.2. Responses with defined policies . . . . . . . . . . . . . 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 . . . . . . . . 29
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Since draft-ietf-httpapi-ratelimit-headers-07 . . . . . . . . . 31
Since draft-ietf-httpapi-ratelimit-headers-03 . . . . . . . . . 31
Since draft-ietf-httpapi-ratelimit-headers-02 . . . . . . . . . 31
Since draft-ietf-httpapi-ratelimit-headers-01 . . . . . . . . . 32
Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
Rate limiting of HTTP clients has become a widespread practice,
especially for HTTP APIs. Typically, servers who do so limit the
number of acceptable requests in a given time window (e.g. 10
requests per second). See Appendix A for further information on the
current usage of rate limiting in HTTP.
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Currently, there is no standard way for servers to communicate quotas
so that clients can throttle their requests to prevent errors. This
document defines a set of standard HTTP header fields to enable rate
limiting:
* RateLimit-Policy: a quota policy, defined by the server, that
client HTTP requests will consume.
* RateLimit: the currently remaining quota available for a specific
policy.
These fields enable establishing complex rate limiting policies,
including using multiple and variable time windows and dynamic
quotas, and implementing concurrency limits.
1.1. Goals
The goals of this document are:
Interoperability: Standardize the names and semantics of rate-limit
headers to ease their enforcement and adoption.
Resiliency: Improve resiliency of HTTP infrastructure by providing
clients with information useful to throttle their requests and
prevent 4xx or 5xx responses.
Documentation: Simplify API documentation by eliminating the need to
include detailed quota limits and related fields in API
documentation.
The following features are out of the scope of this document:
Authorization: RateLimit header fields are not meant to support
authorization or other kinds of access controls.
Response status code: RateLimit header fields may be returned in
both successful (see Section 15.3 of [HTTP]) and non-successful
responses. This specification does not cover whether non
Successful responses count on quota usage, nor does it mandates
any correlation between the RateLimit values and the returned
status code.
Throttling algorithm: This specification does not mandate a specific
throttling algorithm. The values published in the fields,
including the window size, can be statically or dynamically
evaluated.
Service Level Agreement: Conveyed quota hints do not imply any
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service guarantee. Server is free to throttle respectful clients
under certain circumstances.
1.2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The term Origin is to be interpreted as described in Section 7
of[WEB-ORIGIN].
This document uses the terms List, Item and Integer from Section 3 of
[STRUCTURED-FIELDS] to specify syntax and parsing, along with the
concept of "bare item".
2. Terminology
Quota: A quota is an allocation of capacity used by a resource
server to limit client requests. That capacity is measured in
quota units and may be reallocated at the end of a time window.
Quota Unit: A quota unit is the unit of measure used to measure the
activity of a client.
Quota Partition: A quota partition is a division of a server's
capacity across different clients, users and owned resources.
Time Window: A time window indicates a period of time associated to
the allocated quota.
Quota Policy: A quota policy is implemented by the server to
regulate the activity within a specified quota partition,
quantified in quota units, over a defined time window. This
activity is restricted to a predefined limit, known as the quota.
Quota policies can be advertised by servers, but they are not
required to be, and more than one quota policy can affect a given
request from a client to a server.
Service Limit: A service limit is the currently remaining quota from
a specific quota policy and, if defined, the remaining time before
quota is reallocated.
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3. RateLimit-Policy Field
The "RateLimit-Policy" response header field is a non-empty List of
Quota Policy Items (Section 3.1). The Item value MUST be a String.
Its value is informative. The field value is expected to remain
consistent over a the lifetime of a connection. It is this
characteristic that differentiates it from the RateLimit (Section 4)
field that contains information that may change on every request.
RateLimit-Policy: "burst";q=100;w=60,"daily";q=1000;w=86400
3.1. Quota Policy Item
A quota policy Item contains an identifier for the policy and a set
of parameters that contain information about a server's capacity
allocation for the policy.
The following parameters are defined:
q: The REQUIRED "q" parameter indicates the quota allocated by this
policy measured in quota units.
qu: The OPTIONAL "qu" parameter value conveys the quota units
associated to the "q" parameter. The default quota unit is
"requests".
w: The OPTIONAL "w" parameter value conveys a time window.
pk: The OPTIONAL "pk" parameter value conveys the partition key
associated to the corresponding request.
Other parameters are allowed and can be regarded as comments.
Implementation- or service-specific parameters SHOULD be prefixed
parameters with a vendor identifier, e.g. acme-policy, acme-burst.
3.1.1. Quota Parameter
The "q" parameter value MUST be a non-negative Integer. The value
indicates the quota allocated for client activity (measured in quota
units) for a given quota partition.
3.1.2. Quota Unit Parameter
The "qu" parameter value conveys the quota units applicable to the
quota (Section 3.1.1). The value MUST be a String. Allowed values
are listed in the RateLimit Quota Units registry (Section 9.1). This
specification defines three quota units:
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requests: This value indicates the quota is based on the number of
requests processed by the resource server. Whether a specific
request actually consumes a quota unit is implementation-specific.
content-bytes: This value indicates the quota is based on the number
of content bytes processed by the resource server.
concurrent-requests: This value indicates the quota is based on the
number of concurrent requests processed by the resource server.
3.1.3. Window Parameter
The "w" parameter value conveys a time window applicable to the quota
(Section 3.1.1). The time window MUST be a non-negative Integer
value expressing an interval in seconds, similar to the "delay-
seconds" rule defined in Section 10.2.3 of [HTTP]. Sub-second
precision is not supported.
3.1.4. Partition Key Parameter
The "pk" parameter value conveys the partition key associated to the
request. The value MUST be a Byte Sequence. Servers MAY use the
partition key to divide server capacity across different clients and
resources. Quotas are allocated per partition key.
3.2. RateLimit Policy Field Examples
This field MAY convey the time window associated with the quota, as
shown in this example:
RateLimit-Policy: "default";q=100;w=10
These examples show multiple policies being returned:
RateLimit-Policy: "permin";q=50;w=60,"perhr";q=1000;w=3600
The following example shows a policy with a partition key:
RateLimit-Policy: "peruser";q=100;w=60;pk=:cHsdsRa894==:
The following example shows a policy with a partition key and a quota
unit:
RateLimit-Policy: "peruser";q=65535;qu="bytes";w=10;pk=:sdfjLJUOUH==:
This field cannot appear in a trailer section.
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4. RateLimit Field
A server uses the "RateLimit" response header field to communicate
the service limit for a quota policy for a particular partition key.
The field is expressed as List of Service Limit Items (Section 4.1).
RateLimit: "default";r=50;t=30
4.1. Service Limit Item
Each service limit Item identifies the quota policy (Section 3.1)
associated with the request and contains parameters with information
about the current service limit.
The following parameters are defined in this specification:
r: This parameter value conveys the remaining quota units for the
identified policy (Section 4.1.1).
t: This OPTIONAL parameter value conveys the time window reset time
for the identified policy (Section 4.1.2).
pk: The OPTIONAL "pk" parameter value conveys the partition key
associated to the corresponding request.
This field cannot appear in a trailer section. Other parameters are
allowed and can be regarded as comments.
Implementation- or service-specific parameters SHOULD be prefixed
parameters with a vendor identifier, e.g. acme-policy, acme-burst.
4.1.1. Remaining Parameter
The "r" parameter indicates the remaining quota units for the
identified policy (Section 4.1.1).
It is a non-negative Integer expressed in quota units. Clients MUST
NOT assume that a positive remaining value is a guarantee that
further requests will be served. When remaining parameter value is
low, it indicates that the server may soon throttle the client (see
Section 5).
4.1.2. Reset Parameter
The "t" parameter indicates the number of seconds until the quota
associated with the quota policy resets.
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It is a non-negative Integer compatible with the delay-seconds rule,
because:
* it does not rely on clock synchronization and is resilient to
clock adjustment and clock skew between client and server (see
Section 5.6.7 of [HTTP]);
* it mitigates the risk related to thundering herd when too many
clients are serviced with the same timestamp.
The client MUST NOT assume that all its service limit will be reset
at the moment indicated by the reset parameter. The server MAY
arbitrarily alter the reset parameter value between subsequent
requests; for example, in case of resource saturation or to implement
sliding window policies.
4.1.3. Partition Key Parameter
The "pk" parameter value conveys the partition key associated to the
request. The value MUST be a Byte Sequence. Servers MAY use the
partition key to divide server capacity across different clients and
resources. Quotas are allocated per partition key.
4.2. RateLimit Field Examples
This example shows a RateLimit field with a remaining quota of 50
units and a time window reset in 30 seconds:
RateLimit: "default";r=50;t=30
This example shows a remaining quota of 999 requests for a partition
key that has no time window reset:
RateLimit: "default";r=999;pk=:dHJpYWwxMjEzMjM=:
This example shows a 300MB remaining quota for an application in the
next 60 seconds:
RateLimit: "default";r=300000000;t=60;pk=:QXBwLTk5OQ==:
5. Server Behavior
A server MAY return RateLimit header fields independently of the
response status code. This includes on throttled responses. This
document does not mandate any correlation between the RateLimit
header field values and the returned status code.
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Servers should be careful when returning RateLimit header fields in
redirection responses (i.e., responses with 3xx status codes) because
a low remaining parameter 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 parameter value is 0.
HTTP/1.1 301 Moved Permanently
Location: /foo/123
RateLimit: "problemPolicy";r=0;t=10
If a response contains both the Retry-After and the RateLimit header
fields, the reset parameter value SHOULD reference the same point in
time as the Retry-After field value.
A service using RateLimit header fields MUST NOT convey values
exposing an unwanted volume of requests and SHOULD implement
mechanisms to cap the ratio between the remaining and the reset
parameter values (see Section 7.5); this is especially important when
a quota policy uses a large time window.
Under certain conditions, a server MAY artificially lower RateLimit
header field values between subsequent requests, e.g. to respond to
Denial of Service attacks or in case of resource saturation.
5.1. Generating Partition Keys
Servers MAY choose to return partition keys that distinguish between
quota allocated to different consumers or different resources. There
are a wide range of strategies for partitioning server capacity,
including per user, per application, per HTTP method, per resource,
or some combination of those values. The server SHOULD document how
the partition key is generated so that clients can predict the key
value for a future request and determine if there is sufficient quota
remaining to execute the request. Servers should avoid returning
partition keys that contain sensitive information. Servers SHOULD
only use information that is present in the request to generate the
partition key.
5.2. Performance Considerations
Servers are not required to return RateLimit header fields in every
response, and clients need to take this into account. For example,
an implementer concerned with performance might provide RateLimit
header fields only when a given quota is close to exhaustion.
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Implementers concerned with response fields' size, might take into
account their ratio with respect to the content length, or use
header-compression HTTP features such as [HPACK].
6. Client Behavior
The RateLimit header fields can be used by clients to determine
whether the associated request respected the server's quota policy,
and as an indication of whether subsequent requests will. However,
the server might apply other criteria when servicing future requests,
and so the quota policy may not completely reflect whether requests
will succeed.
For example, a successful response with the following fields:
RateLimit: "default";r=1;t=7
does not guarantee that the next request will be successful.
Servers' behavior may be subject to other conditions.
A client is responsible for ensuring that RateLimit header field
values returned cause reasonable client behavior with respect to
throughput and latency (see Section 7.5 and Section 7.5.1).
A client receiving RateLimit header fields MUST NOT assume that
future responses will contain the same RateLimit header fields, or
any RateLimit header fields at all.
Malformed RateLimit header fields MUST be ignored.
A client SHOULD NOT exceed the quota units conveyed by the remaining
parameter before the time window expressed in the reset parameter.
The value of the reset parameter 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 parameter
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 parameter 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 pre-emptively back-off their request rate when
approaching quota limits;
* consuming all the quota according to the exposed limits and then
wait.
6.1. Consuming Partition Keys
Partition keys are useful for a client if it is likely that single
client will make requests that consume different quota allocations.
E.g. a client making requests on behalf of different users or for
different resources that have independent quota allocations.
If a server documents the partition key generation algorithm, clients
MAY generate a partition key for a future request. Using this key,
and comparing to the key returned by the server, the client can
determine if there is sufficient quota remaining to execute the
request.
For cases where the partition key generation algorithm of a server is
unknown, clients MAY use heuristics to guess if a future request will
be successful based on its similarity to previous requests.
6.2. 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.
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This specification does not mandate any behavior on intermediaries
respect to retries, nor requires that intermediaries have any role in
respecting quota policies. For example, it is legitimate for a proxy
to retransmit a request without notifying the client, and thus
consuming quota units.
Privacy considerations (Section 8) provide further guidance on
intermediaries.
6.3. Caching
[HTTP-CACHING] defines how responses can be stored and reused for
subsequent requests, including those with RateLimit header fields.
Because the information in RateLimit header fields on a cached
response may not be current, they SHOULD be ignored on responses that
come from cache (i.e., those with a positive current_age; see
Section 4.2.3 of [HTTP-CACHING]).
7. Security Considerations
7.1. Throttling does not prevent clients from issuing requests
This specification does not prevent clients from making requests.
Servers should always implement mechanisms to prevent resource
exhaustion.
7.2. Information disclosure
Servers should not disclose to untrusted parties operational capacity
information that can be used to saturate its infrastructural
resources.
While this specification does not mandate whether non-successful
responses consume quota, if error responses (such as 401
(Unauthorized) and 403 (Forbidden)) count against quota, a malicious
client could probe the endpoint to get traffic information of another
user.
As intermediaries might retransmit requests and consume quota units
without prior knowledge of the user agent, RateLimit header fields
might reveal the existence of an intermediary to the user agent.
Where partition keys contain identifying information, either of the
client application or the user, servers should be aware of the
potential for impersonation and apply the appropriate security
mechanisms.
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7.3. Remaining quota units are not granted requests
RateLimit header fields convey hints from the server to the clients
in order to help them avoid being throttled out.
Clients MUST NOT consider the quota returned in the remaining
parameter (Section 4.1.1) as a service level agreement.
In case of resource saturation, the server MAY artificially lower the
returned values or not serve the request regardless of the advertised
quotas.
7.4. Reliability of the reset parameter
Consider that quota might not be restored after the moment referenced
by the reset parameter (Section 4.1.2), and the reset parameter value
may not be constant.
Subsequent requests might return a higher reset parameter value to
limit concurrency or implement dynamic or adaptive throttling
policies.
7.5. Resource exhaustion
When returning reset values, servers must be aware that many
throttled clients may come back at the very moment specified.
This is true for Retry-After too.
For example, if the quota resets every day at 18:00:00 and your
server returns the reset parameter accordingly
Date: Tue, 15 Nov 1994 18:00:00 GMT
RateLimit: "daily";r=1;t=36400
there's a high probability that all clients will show up at 18:00:00.
This could be mitigated by adding some jitter to the reset value.
Resource exhaustion issues can be associated with quota policies
using a large time window, because a user agent by chance or on
purpose might consume most of its quota units in a significantly
shorter interval.
This behavior can be even triggered by the provided RateLimit header
fields. The following example describes a service with an unconsumed
quota policy of 10000 quota units per 1000 seconds.
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RateLimit-Policy: "somepolicy";q=10000;w=1000
RateLimit: "somepolicy";r=10000;t=10
A client implementing a simple ratio between remaining parameter and
reset parameter could infer an average throughput of 1000 quota units
per second, while the quota parameter 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 parameter value
or an higher reset parameter value. Moreover, complementing large
time window quota policies with a short time window one mitigates
those risks.
7.5.1. Denial of Service
RateLimit header fields may contain unexpected values by chance or on
purpose. For example, an excessively high remaining parameter value
may be:
* used by a malicious intermediary to trigger a Denial of Service
attack or consume client resources boosting its requests;
* passed by a misconfigured server;
or a high reset parameter 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
parameter exceeds ten minutes.
The considerations above are not limited to RateLimit header fields,
but apply to all fields affecting how clients behave in subsequent
requests (e.g. Retry-After).
8. Privacy Considerations
Clients that act upon a request to rate limit are potentially re-
identifiable (see Section 5.2.1 of [PRIVACY]) because they react to
information that might only be given to them. Note that this might
apply to other fields too (e.g. Retry-After).
Since rate limiting is usually implemented in contexts where clients
are either identified or profiled (e.g. assigning different quota
units to different users), this is rarely a concern.
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Privacy enhancing infrastructures using RateLimit header fields can
define specific techniques to mitigate the risks of re-
identification.
9. IANA Considerations
IANA is requested to update one registry and create one new registry.
Please add the following entries to the "Hypertext Transfer Protocol
(HTTP) Field Name Registry" registry ([HTTP]):
+==================+===========+=======================+
| Field Name | Status | Specification |
+==================+===========+=======================+
| RateLimit | permanent | Section 4 of RFC nnnn |
+------------------+-----------+-----------------------+
| RateLimit-Policy | permanent | Section 3 of RFC nnnn |
+------------------+-----------+-----------------------+
Table 1
9.1. RateLimit quota unit registry
This specification establishes the registry "Hypertext Transfer
Protocol (HTTP) RateLimit Quota Units" registry to be located at
https://www.iana.org/assignments/http-ratelimit-quota-units.
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).
The registry has the following initial content:
+===============+===========+=======+
| Quota Unit | Reference | Notes |
+===============+===========+=======+
| request | RFC nnnn | |
+---------------+-----------+-------+
| content-bytes | RFC nnnn | |
+---------------+-----------+-------+
Table 2
9.1.1. Registration Template
The registration template for the RateLimit Quota Units registry is
as follows:
* Quota Unit: The name of the quota unit.
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* Reference: A reference to the document that specifies the quota
unit.
* Notes: Any additional notes about the quota unit.
10. References
10.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<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>.
[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>.
10.2. 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>.
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[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.
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.
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Moreover system metrics, statistics and heuristics can be used to
implement more complex policies, where the number of acceptable
requests and the time window are computed dynamically.
To help clients throttling their requests, servers may expose the
counters used to evaluate quota policies via HTTP header fields.
Those response headers may be added by HTTP intermediaries such as
API gateways and reverse proxies.
On the web we can find many different rate-limit headers, usually
containing the number of allowed requests in a given time window, and
when the window is reset.
The common choice is to return three headers containing:
* the maximum number of allowed requests in the time window;
* the number of remaining requests in the current window;
* the time remaining in the current window expressed in seconds or
as a timestamp;
A.1. Interoperability issues
A major interoperability issue in throttling is the lack of standard
headers, because:
* each implementation associates different semantics to the same
header field names;
* header field names proliferates.
User agents interfacing with different servers may thus need to
process different headers, or the very same application interface
that sits behind different reverse proxies may reply with different
throttling headers.
Appendix B. Examples
B.1. Responses without defining policies
Some servers may not expose the policy limits in the RateLimit-Policy
header field. Clients can still use the RateLimit header field to
throttle their requests.
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B.1.1. Throttling information in responses
The client exhausted its quota for the next 50 seconds. The limit
and time-window is communicated out-of-band.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: "default";r=0;t=50
{"hello": "world"}
Since the field values are not necessarily correlated with the
response status code, a subsequent request is not required to fail.
The example below shows that the server decided to serve the request
even if remaining parameter value is 0. Another server, or the same
server under other load conditions, could have decided to throttle
the request instead.
Request:
GET /items/456 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: "default";r=0;t=48
{"still": "successful"}
B.1.2. Multiple policies in response
The server uses two different policies to limit the client's
requests:
* 5000 daily quota units;
* 1000 hourly quota units.
The client consumed 4900 quota units in the first 14 hours.
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Despite the next hourly limit of 1000 quota units, the closest limit
to reach is the daily one.
The server then exposes the RateLimit header fields to inform the
client that:
* it has only 100 quota units left in the daily quota and the window
will reset in 10 hours;
The server MAY choose to omit returning the hourly policy as it uses
the same quota units as the daily policy and the daily policy is the
one that is closest to being exhausted.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: "dayLimit";r=100;t=36000
{"hello": "world"}
B.1.3. Use for limiting concurrency
RateLimit header fields may be used to limit concurrency, advertising
limits that are lower than the usual ones in case of saturation, thus
increasing availability.
The server adopted a basic policy of 100 quota units per minute, and
in case of resource exhaustion adapts the returned values reducing
both limit and remaining parameter values.
After 2 seconds the client consumed 40 quota units
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
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HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Policy: "basic";q=100;w=60
RateLimit: "basic";r=60;t=58
{"elapsed": 2, "issued": 40}
At the subsequent request - due to resource exhaustion - the server
advertises only r=20.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Policy: "basic";q=100;w=60
RateLimit: "basic";r=20;t=56
{"elapsed": 4, "issued": 41}
B.1.4. Use in throttled responses
A client exhausted its quota and the server throttles it sending
Retry-After.
In this example, the values of Retry-After and RateLimit header field
reference the same moment, but this is not a requirement.
The 429 (Too Many Request) HTTP status code is just used as an
example.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
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HTTP/1.1 429 Too Many Requests
Content-Type: application/json
Date: Mon, 05 Aug 2019 09:27:00 GMT
Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
RateLimit: "default";r=0;t=5
{
"title": "Too Many Requests",
"status": 429,
"detail": "You have exceeded your quota"
}
B.2. Responses with defined policies
B.2.1. Throttling window specified via parameter
The client has 99 quota units left for the next 50 seconds. The time
window is communicated by the w parameter, so we know the throughput
is 100 quota units per minute.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit: "fixedwindow";r=99;t=50
RateLimit-Policy: "fixedwindow";q=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 parameter then advertises only 9 quota units for the
next 50 seconds to slow down the client.
Note that the server could have lowered even the other values in the
RateLimit header field: this specification does not mandate any
relation between the field values contained in subsequent responses.
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Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Policy: "dynamic";q=100;w=60
RateLimit: "dynamic";r=9;t=50
{
"status": 200,
"detail": "Just slow down without waiting."
}
B.2.3. Dynamic limits for pushing back and slowing down
Continuing the previous example, let's say the client waits 10
seconds and performs a new request which, due to resource exhaustion,
the server rejects and pushes back, advertising r=0 for the next 20
seconds.
The server advertises a smaller window with a lower limit to slow
down the client for the rest of its original window after the 20
seconds elapse.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 429 Too Many Requests
Content-Type: application/json
RateLimit-Policy: "dynamic";q=15;w=20
RateLimit: "dynamic";r=0;t=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-Policy: "dynamic";q=100;w=60
RateLimit: "dynamic";r=15;t=40
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
Note that in this last response the client is expected to honor
Retry-After and perform no requests for the specified amount of time,
whereas the previous example would not force the client to stop
requests before the reset time is elapsed, as it would still be free
to query again the server even if it is likely to have the request
rejected.
B.3.1. Missing Remaining information
The server does not expose remaining values (for example, because the
underlying counters are not available). Instead, it resets the limit
counter every second.
It communicates to the client the limit of 10 quota units per second
always returning the limit and reset parameters.
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-Policy: quota;q=100;w=1
RateLimit: quota;t=1
{"first": "request"}
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Policy: quota;q=10
RateLimit: quota;t=1
{"second": "request"}
B.3.2. Use with multiple windows
This is a standardized way of describing the policy detailed in
Appendix B.1.2:
* 5000 daily quota units;
* 1000 hourly quota units.
The client consumed 4900 quota units in the first 14 hours.
Despite the next hourly limit of 1000 quota units, the closest limit
to reach is the daily one.
The server then exposes the RateLimit header fields to inform the
client that:
* it has only 100 quota units left;
* the window will reset in 10 hours;
* the expiring-limit is 5000.
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Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 OK
Content-Type: application/json
RateLimit-Policy: "hour";q=1000;w=3600, "day";q=5000;w=86400
RateLimit: "day";r=100;t=36000
{"hello": "world"}
FAQ
This section is to be removed before publishing as an RFC.
1. Why defining standard fields for throttling?
To simplify enforcement of throttling policies and enable clients
to constraint their requests to avoid being throttled.
2. Can I use RateLimit header fields in throttled responses (eg with
status code 429)?
Yes, you can.
3. Are those specs tied to RFC 6585?
No. [RFC6585] defines the 429 status code and we use it just as
an example of a throttled request, that could instead use even
403 or whatever status code.
4. Why is the partition key necessary?
Without a partition key, a server can only effectively only have
one scope (aka partition), which is impractical for most
services, or it needs to communicate the scopes out-of-band.
This prevents the development of generic connector code that can
be used to prevent requests from being throttled. Many APIs rely
on API keys, user identity or client identity to allocate quota.
As soon as a single client processes requests for more than one
partition, the client needs to know the corresponding partition
key to properly track requests against allocated quota.
5. Why using delay-seconds instead of a UNIX Timestamp? Why not
using subsecond precision?
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Using delay-seconds aligns with Retry-After, which is returned in
similar contexts, eg on 429 responses.
Timestamps require a clock synchronization protocol (see
Section 5.6.7 of [HTTP]). This may be problematic (e.g. clock
adjustment, clock skew, failure of hardcoded clock
synchronization servers, IoT devices, ..). Moreover timestamps
may not be monotonically increasing due to clock adjustment. See
Another NTP client failure story
(https://community.ntppool.org/t/another-ntp-client-failure-
story/1014/)
We did not use subsecond precision because:
* that is more subject to system clock correction like the one
implemented via the adjtimex() Linux system call;
* response-time latency may not make it worth. A brief
discussion on the subject is on the httpwg ml
(https://lists.w3.org/Archives/Public/ietf-http-
wg/2019JulSep/0202.html)
* almost all rate-limit headers implementations do not use it.
6. Shouldn't I limit concurrency instead of request rate?
You can use this specification to limit concurrency at the HTTP
level (see {#use-for-limiting-concurrency}) and help clients to
shape their requests avoiding being throttled out.
A problematic way to limit concurrency is connection dropping,
especially when connections are multiplexed (e.g. HTTP/2)
because this results in unserviced client requests, which is
something we want to avoid.
A semantic way to limit concurrency is to return 503 + Retry-
After in case of resource saturation (e.g. thrashing, connection
queues too long, Service Level Objectives not meet, ..).
Saturation conditions can be either dynamic or static: all this
is out of the scope for the current document.
7. Do a positive value of remaining paramter imply any service
guarantee for my future requests to be served?
No. FAQ integrated in Section 4.1.1.
8. Is the quota-policy definition too complex?
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You can always return the simplest form
RateLimit:"default";r=50;t=60
The policy key clearly connects the current usage status of a policy
to the defined limits. So for the following field:
RateLimit-Policy: "sliding";q=100;w=60;burst=1000
RateLimit-Policy: "fixed";q=5000;w=3600;burst=0
RateLimit: "sliding";r=50;t=44
the value "sliding" identifies the policy being reported.
1. Can intermediaries alter RateLimit header fields?
Generally, they should not because it might result in unserviced
requests. There are reasonable use cases for intermediaries
mangling RateLimit header fields though, e.g. when they enforce
stricter quota-policies, or when they are an active component of
the service. In those case we will consider them as part of the
originating infrastructure.
2. Why the w parameter is just informative? Could it be used by a
client to determine the request rate?
A non-informative w parameter might be fine in an environment
where clients and servers are tightly coupled. Conveying
policies with this detail on a large scale would be very complex
and implementations would be likely not interoperable. We thus
decided to leave w as an informational parameter and only rely on
the limit, remaining and reset parameters for defining the
throttling behavior.
3. Can I use RateLimit fields in trailers? Servers usually
establish whether the request is in-quota before creating a
response, so the RateLimit field values should be already
available in that moment. Supporting trailers has the only
advantage that allows to provide more up-to-date information to
the client in case of slow responses. However, this complicates
client implementations with respect to combining fields from
headers and accounting for intermediaries that drop trailers.
Since there are no current implementations that use trailers, we
decided to leave this as a future-work.
RateLimit header fields currently used on the web
This section is to be removed before publishing as an RFC.
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Commonly used header field names are:
* X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset;
There are variants too, where the window is specified in the header
field name, eg:
* x-ratelimit-limit-minute, x-ratelimit-limit-hour, x-ratelimit-
limit-day
* x-ratelimit-remaining-minute, x-ratelimit-remaining-hour, x-
ratelimit-remaining-day
Here are some interoperability issues:
* X-RateLimit-Remaining references different values, depending on
the implementation:
- seconds remaining to the window expiration
- milliseconds remaining to the window expiration
- seconds since UTC, in UNIX Timestamp [UNIX]
- a datetime, either IMF-fixdate [HTTP] or [RFC3339]
* different headers, with the same semantic, are used by different
implementers:
- X-RateLimit-Limit and X-Rate-Limit-Limit
- X-RateLimit-Remaining and X-Rate-Limit-Remaining
- X-RateLimit-Reset and X-Rate-Limit-Reset
The semantic of RateLimit depends on the windowing algorithm. A
sliding window policy for example, may result in having a remaining
parameter value related to the ratio between the current and the
maximum throughput. e.g.
RateLimit-Policy: "sliding";q=12;w=1
; using 50% of throughput, that is 6 units/s
RateLimit: "sliding";q=12;r=6;t=1
If this is the case, the optimal solution is to achieve
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RateLimit-Policy: "sliding";q=12;w=1
; using 100% of throughput, that is 12 units/s
RateLimit: "sliding";q=12;r=1;t=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,
and Julian Reschke.
Changes
This section is to be removed before publishing as an RFC.
Since draft-ietf-httpapi-ratelimit-headers-07
This section is to be removed before publishing as an RFC.
* Refactored both fields to lists of Items that identify policy and
use parameters
* Added quota unit parameter
* Added partition key parameter
Since draft-ietf-httpapi-ratelimit-headers-03
This section is to be removed before publishing as an RFC.
* Split policy informatiom in RateLimit-Policy #81
Since draft-ietf-httpapi-ratelimit-headers-02
This section is to be removed before publishing as an RFC.
* Address throttling scope #83
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Since draft-ietf-httpapi-ratelimit-headers-01
This section is to be removed before publishing as an RFC.
* Update IANA considerations #60
* Use Structured fields #58
* Reorganize document #67
Since draft-ietf-httpapi-ratelimit-headers-00
This section is to be removed before publishing as an RFC.
* Use I-D.httpbis-semantics, which includes referencing delay-
seconds instead of delta-seconds. #5
Authors' Addresses
Roberto Polli
Team Digitale, Italian Government
Italy
Email: robipolli@gmail.com
Alejandro Martinez Ruiz
Red Hat
Email: alex@flawedcode.org
Darrel Miller
Microsoft
Email: darrel@tavis.ca
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