HTTPAPI R. Polli
Internet-Draft Team Digitale, Italian Government
Intended status: Standards Track A. Martinez
Expires: 12 November 2021 Red Hat
11 May 2021
RateLimit Header Fields for HTTP
draft-ietf-httpapi-ratelimit-headers-01
Abstract
This document defines the RateLimit-Limit, RateLimit-Remaining,
RateLimit-Reset fields for HTTP, thus allowing servers to publish
current service limits and clients to shape their request policy and
avoid being throttled out.
Note to Readers
_RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP working group
mailing list (httpapi@ietf.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-httpapi-wg/
(https://lists.w3.org/Archives/Public/ietf-httpapi-wg/).
The source code and issues list for this draft can be found at
https://github.com/ietf-wg-httpapi/ratelimit-headers
(https://github.com/ietf-wg-httpapi/ratelimit-headers).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 November 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Rate-limiting and quotas . . . . . . . . . . . . . . . . 3
1.2. Current landscape of rate-limiting headers . . . . . . . 4
1.2.1. Interoperability issues . . . . . . . . . . . . . . . 4
1.3. This proposal . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5. Notational Conventions . . . . . . . . . . . . . . . . . 6
2. Expressing rate-limit policies . . . . . . . . . . . . . . . 6
2.1. Time window . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Service limit . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Quota policy . . . . . . . . . . . . . . . . . . . . . . 7
3. Header Specifications . . . . . . . . . . . . . . . . . . . . 8
3.1. RateLimit-Limit . . . . . . . . . . . . . . . . . . . . . 8
3.2. RateLimit-Remaining . . . . . . . . . . . . . . . . . . . 9
3.3. RateLimit-Reset . . . . . . . . . . . . . . . . . . . . . 9
4. Providing RateLimit fields . . . . . . . . . . . . . . . . . 10
4.1. Performance considerations . . . . . . . . . . . . . . . 11
5. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . 12
6. Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Receiving RateLimit fields . . . . . . . . . . . . . . . . . 12
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Unparameterized responses . . . . . . . . . . . . . . . . 13
8.1.1. Throttling information in responses . . . . . . . . . 13
8.1.2. Use in conjunction with custom fields . . . . . . . . 14
8.1.3. Use for limiting concurrency . . . . . . . . . . . . 15
8.1.4. Use in throttled responses . . . . . . . . . . . . . 16
8.2. Parameterized responses . . . . . . . . . . . . . . . . . 17
8.2.1. Throttling window specified via parameter . . . . . . 17
8.2.2. Dynamic limits with parameterized windows . . . . . . 17
8.2.3. Dynamic limits for pushing back and slowing down . . 18
8.3. Dynamic limits for pushing back with Retry-After and slow
down . . . . . . . . . . . . . . . . . . . . . . . . . . 19
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8.3.1. Missing Remaining information . . . . . . . . . . . . 20
8.3.2. Use with multiple windows . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9.1. Throttling does not prevent clients from issuing
requests . . . . . . . . . . . . . . . . . . . . . . . . 21
9.2. Information disclosure . . . . . . . . . . . . . . . . . 21
9.3. Remaining quota-units are not granted requests . . . . . 22
9.4. Reliability of RateLimit-Reset . . . . . . . . . . . . . 22
9.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 22
9.6. Denial of Service . . . . . . . . . . . . . . . . . . . . 23
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10.1. RateLimit-Limit Field Registration . . . . . . . . . . . 23
10.2. RateLimit-Remaining Field Registration . . . . . . . . . 24
10.3. RateLimit-Reset Field Registration . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Appendix B. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 25
RateLimit fields currently used on the web . . . . . . . . . . . 29
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
The widespreading of HTTP as a distributed computation protocol
requires an explicit way of communicating service status and usage
quotas.
This was partially addressed by the "Retry-After" header field
defined in [SEMANTICS] to be returned in "429 Too Many Requests" (see
[STATUS429]) or "503 Service Unavailable" responses.
Still, there is not a standard way to communicate service quotas so
that the client can throttle its requests and prevent 4xx or 5xx
responses.
1.1. 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 - eg. monetization.
A basic quota mechanism limits the number of acceptable requests in a
given time window, eg. 10 requests per second.
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When quota is exceeded, servers usually do not serve the request
replying instead with a "4xx" HTTP status code (eg. 429 or 403) or
adopt more aggressive policies like dropping connections.
Quotas may be enforced on different basis (eg. 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 request per minute;
* limited to 1000 request per hour.
Moreover system metrics, statistics and heuristics can be used to
implement more complex policies, where the number of acceptable
request and the time window are computed dynamically.
1.2. Current landscape of rate-limiting headers
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;
1.2.1. Interoperability issues
A major interoperability issue in throttling is the lack of standard
headers, because:
* each implementation associates different semantics to the same
header field names;
* header field names proliferates.
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User Agents interfacing with different servers may thus need to
process different headers, or the very same application interface
that sits behind different reverse proxies may reply with different
throttling headers.
1.3. This proposal
This proposal defines syntax and semantics for the following fields:
* "RateLimit-Limit": containing the requests quota in the time
window;
* "RateLimit-Remaining": containing the remaining requests quota in
the current window;
* "RateLimit-Reset": containing the time remaining in the current
window, specified in seconds.
The behavior of "RateLimit-Reset" is compatible with the "delay-
seconds" notation of "Retry-After".
The fields definition allows to describe complex policies, including
the ones using multiple and variable time windows and dynamic quotas,
or implementing concurrency limits.
1.4. Goals
The goals of this proposal are:
1. Standardizing the names and semantics of rate-limit headers to
ease their enforcement and adoption;
2. Improve resiliency of HTTP infrastructure by providing clients
with information useful to throttle their requests and prevent
4xx or 5xx responses;
3. Simplify API documentation by eliminating the need to include
detailed quota limits and related header fields in API
documentation.
The goals do not include:
Authorization: The rate-limit fields described here are not meant to
support authorization or other kinds of access controls.
Throttling scope: This specification does not cover the throttling
scope, that may be the given resource-target, its parent path or
the whole Origin (see Section 7 of [RFC6454]).
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Response status code: The rate-limit fields may be returned in both
successful (see Section 15.3 of [SEMANTICS]) and non-successful
responses. This specification does not cover whether non
Successful responses count on quota usage, nor it mandates any
correlation between the RateLimit values and the returned status
code.
Throttling policy: This specification does not mandate a specific
throttling policy. The values published in the fields, including
the window size, can be statically or dynamically evaluated.
Service Level Agreement: Conveyed quota hints do not imply any
service guarantee. Server is free to throttle respectful clients
under certain circumstances.
1.5. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the Augmented BNF defined in [RFC5234] and updated
by [RFC7405] along with the "#rule" extension defined in
Section 5.6.1 of [SEMANTICS].
The term Origin is to be interpreted as described in Section 7 of
[RFC6454].
The "delay-seconds" rule is defined in Section 10.2.4 of [SEMANTICS].
2. Expressing rate-limit policies
2.1. Time window
Rate limit policies limit the number of acceptable requests in a
given time window.
A time window is expressed in seconds, using the following syntax:
time-window = delay-seconds
Subsecond precision is not supported.
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2.2. Service limit
The service-limit is a value associated to the maximum number of
requests that the server is willing to accept from one or more
clients on a given basis (originating IP, authenticated user,
geographical, ..) during a "time-window" as defined in Section 2.1.
The "service-limit" is expressed in "quota-units" and has the
following syntax:
service-limit = quota-units
quota-units = 1*DIGIT
The "service-limit" SHOULD match the maximum number of acceptable
requests.
The "service-limit" MAY differ from the total number of acceptable
requests when weight mechanisms, bursts, or other server policies are
implemented.
If the "service-limit" does not match the maximum number of
acceptable requests the relation with that SHOULD be communicated
out-of-band.
Example: A server could
* count once requests like "/books/{id}"
* count twice search requests like "/books?author=Camilleri"
so that we have the following counters
GET /books/123 ; service-limit=4, remaining: 3, status=200
GET /books?author=Camilleri ; service-limit=4, remaining: 1, status=200
GET /books?author=Eco ; service-limit=4, remaining: 0, status=429
2.3. Quota policy
This specification allows describing a quota policy with the
following syntax:
quota-policy = service-limit; "w" "=" time-window
*( OWS ";" OWS quota-comment)
quota-comment = token "=" (token / quoted-string)
quota-policy parameters like "w" and quota-comment tokens MUST NOT
occur multiple times within the same quota-policy.
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An example policy of 100 quota-units per minute.
100;w=60
The definition of a quota-policy does not imply any specific
distribution of quota-units over time. Such service specific details
can be conveyed in the "quota-comments".
Two examples of providing further details via custom parameters in
"quota-comments".
100;w=60;comment="fixed window"
12;w=1;burst=1000;policy="leaky bucket"
3. Header Specifications
The following "RateLimit" response fields are defined
3.1. RateLimit-Limit
The "RateLimit-Limit" response field indicates the "service-limit"
associated to the client in the current "time-window".
If the client exceeds that limit, it MAY not be served.
The field value is
RateLimit-Limit = expiring-limit [, 1#quota-policy ]
expiring-limit = service-limit
The "expiring-limit" value MUST be set to the "service-limit" that is
closer to reach its limit.
The "quota-policy" is defined in Section 2.3, and its values are
informative.
RateLimit-Limit: 100
A "time-window" associated to "expiring-limit" can be communicated
via an optional "quota-policy" value, like shown in the following
example
RateLimit-Limit: 100, 100;w=10
If the "expiring-limit" is not associated to a "time-window", the
"time-window" MUST either be:
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* inferred by the value of "RateLimit-Reset" at the moment of the
reset, or
* communicated out-of-band (eg. in the documentation).
Policies using multiple quota limits MAY be returned using multiple
"quota-policy" items, like shown in the following two examples:
RateLimit-Limit: 10, 10;w=1, 50;w=60, 1000;w=3600, 5000;w=86400
RateLimit-Limit: 10, 10;w=1;burst=1000, 1000;w=3600
This field MUST NOT occur multiple times and can be sent in a trailer
section.
3.2. RateLimit-Remaining
The "RateLimit-Remaining" response field indicates the remaining
"quota-units" defined in Section 2.2 associated to the client.
The field value is
RateLimit-Remaining = quota-units
This field MUST NOT occur multiple times and can be sent in a trailer
section.
Clients MUST NOT assume that a positive "RateLimit-Remaining" value
is a guarantee that further requests will be served.
A low "RateLimit-Remaining" value is like a yellow traffic-light for
either the number of requests issued in the "time-window" or the
request throughput: the red light may arrive suddenly (see
Section 4).
One example of "RateLimit-Remaining" use is below.
RateLimit-Remaining: 50
3.3. RateLimit-Reset
The "RateLimit-Reset" response field indicates either
* the number of seconds until the quota resets.
The field value is
RateLimit-Reset = delay-seconds
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The delay-seconds format is used because:
* it does not rely on clock synchronization and is resilient to
clock adjustment and clock skew between client and server (see
Section 5.6.7 of [SEMANTICS]);
* it mitigates the risk related to thundering herd when too many
clients are serviced with the same timestamp.
This field MUST NOT occur multiple times and can be sent in a trailer
section.
An example of "RateLimit-Reset" use is below.
RateLimit-Reset: 50
The client MUST NOT assume that all its "service-limit" will be
restored after the moment referenced by "RateLimit-Reset". The
server MAY arbitrarily alter the "RateLimit-Reset" value between
subsequent requests eg. in case of resource saturation or to
implement sliding window policies.
4. Providing RateLimit fields
A server MAY use one or more "RateLimit" response fields defined in
this document to communicate its quota policies.
The returned values refers to the metrics used to evaluate if the
current request respects the quota policy and MAY not apply to
subsequent requests.
Example: a successful response with the following fields
RateLimit-Limit: 10
RateLimit-Remaining: 1
RateLimit-Reset: 7
does not guarantee that the next request will be successful. Server
metrics may be subject to other conditions like the one shown in the
example from Section 2.2.
A server MAY return "RateLimit" response fields independently of the
response status code. This includes throttled responses.
This document does not mandate any correlation between the
"RateLimit" values and the returned status code.
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Servers should be careful in returning "RateLimit" fields in
redirection responses (eg. 3xx status codes) because a low
"RateLimit-Remaining" value could limit the client from issuing
requests. For example, given the rate limiting fields below, a
client could decide to wait 10 seconds before following the
"Location" header, because "RateLimit-Remaining" is 0.
HTTP/1.1 301 Moved Permanently
Location: /foo/123
RateLimit-Remaining: 0
RateLimit-Limit: 10
RateLimit-Reset: 10
If a response contains both the "Retry-After" and the "RateLimit-
Reset" fields, the value of "RateLimit-Reset" SHOULD reference the
same point in time as "Retry-After".
When using a policy involving more than one "time-window", the server
MUST reply with the "RateLimit" fields related to the window with the
lower "RateLimit-Remaining" values.
A service returning "RateLimit" fields MUST NOT convey values
exposing an unwanted volume of requests and SHOULD implement
mechanisms to cap the ratio between "RateLimit-Remaining" and
"RateLimit-Reset" (see Section 9.5); this is especially important
when quota-policies use a large "time-window".
Under certain conditions, a server MAY artificially lower "RateLimit"
field values between subsequent requests, eg. to respond to Denial of
Service attacks or in case of resource saturation.
Servers usually establish whether the request is in-quota before
creating a response, so the RateLimit field values should be already
available in that moment. Nonetheless servers MAY decide to send the
"RateLimit" fields in a trailer section.
4.1. Performance considerations
Servers are not required to return "RateLimit" fields in every
response, and clients need to take this into account. For example,
an implementer concerned with performance might provide "RateLimit"
fields only when a given quota is going to expire.
Implementers concerned with response fields' size, might take into
account their ratio with respect to the payload data, or use header-
compression http features such as [HPACK].
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5. Intermediaries
This section documents the considerations advised in Section 16.3.3
of [SEMANTICS].
An intermediary that is not part of the originating service
infrastructure and is not aware of the quota-policy semantic used by
the Origin Server SHOULD NOT alter the RateLimit fields' values in
such a way as to communicate a more permissive quota-policy; this
includes removing the RateLimit fields.
An intermediary MAY alter the RateLimit fields in such a way as to
communicate a more restrictive quota-policy when:
* it is aware of the quota-unit semantic used by the Origin Server;
* it implements this specification and enforces a quota-policy which
is more restrictive than the one conveyed in the fields.
An intermediary SHOULD forward a request even when presuming that it
might not be serviced; the service returning the RateLimit fields is
the sole responsible of enforcing the communicated quota-policy, and
it is always free to service incoming requests.
This specification does not mandate any behavior on intermediaries
respect to retries, nor requires that intermediaries have any role in
respecting quota-policies. For example, it is legitimate for a proxy
to retransmit a request without notifying the client, and thus
consuming quota-units.
6. Caching
As is the ordinary case for HTTP caching ([RFC7234]), a response with
RateLimit fields might be cached and re-used for subsequent requests.
A cached "RateLimit" response does not modify quota counters but
could contain stale information. Clients interested in determining
the freshness of the "RateLimit" fields could rely on fields such as
"Date" and on the "time-window" of a "quota-policy".
7. Receiving RateLimit fields
A client MUST process the received "RateLimit" fields.
A client MUST validate the values received in the "RateLimit" fields
before using them and check if there are significant discrepancies
with the expected ones. This includes a "RateLimit-Reset" moment too
far in the future or a "service-limit" too high.
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A client receiving "RateLimit" fields MUST NOT assume that subsequent
responses contain the same "RateLimit" fields, or any "RateLimit"
fields at all.
Malformed "RateLimit" fields MAY be ignored.
A client SHOULD NOT exceed the "quota-units" expressed in "RateLimit-
Remaining" before the "time-window" expressed in "RateLimit-Reset".
A client MAY still probe the server if the "RateLimit-Reset" is
considered too high.
The value of "RateLimit-Reset" is generated at response time: a
client aware of a significant network latency MAY behave accordingly
and use other information (eg. the "Date" response header field, or
otherwise gathered metrics) to better estimate the "RateLimit-Reset"
moment intended by the server.
The "quota-policy" values and comments provided in "RateLimit-Limit"
are informative and MAY be ignored.
If a response contains both the "RateLimit-Reset" and "Retry-After"
fields, "Retry-After" MUST take precedence and "RateLimit-Reset" MAY
be ignored.
This specification does not mandate a specific throttling behavior
and implementers can adopt their preferred policies, including:
* slowing down or preemptively backoff their request rate when
approaching quota limits;
* consuming all the quota according to the exposed limits and then
wait.
8. Examples
8.1. Unparameterized responses
8.1.1. 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
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Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
Ratelimit-Remaining: 0
Ratelimit-Reset: 50
{"hello": "world"}
Since the field values are not necessarily correlated with the
response status code, a subsequent request is not required to fail.
The example below shows that the server decided to serve the request
even if "RateLimit-Remaining" is 0. Another server, or the same
server under other load conditions, could have decided to throttle
the request instead.
Request:
GET /items/456 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
Ratelimit-Remaining: 0
Ratelimit-Reset: 48
{"still": "successful"}
8.1.2. Use in conjunction with custom fields
The server uses two custom fields, namely "acme-RateLimit-DayLimit"
and "acme-RateLimit-HourLimit" to expose the following policy:
* 5000 daily quota-units;
* 1000 hourly quota-units.
The client consumed 4900 quota-units in the first 14 hours.
Despite the next hourly limit of 1000 quota-units, the closest limit
to reach is the daily one.
The server then exposes the "RateLimit-*" fields to inform the client
that:
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* it has only 100 quota-units left;
* the window will reset in 10 hours.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
acme-RateLimit-DayLimit: 5000
acme-RateLimit-HourLimit: 1000
RateLimit-Limit: 5000
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"}
8.1.3. Use for limiting concurrency
Throttling fields may be used to limit concurrency, advertising
limits that are lower than the usual ones in case of saturation, thus
increasing availability.
The server adopted a basic policy of 100 quota-units per minute, and
in case of resource exhaustion adapts the returned values reducing
both "RateLimit-Limit" and "RateLimit-Remaining".
After 2 seconds the client consumed 40 quota-units
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 60
RateLimit-Reset: 58
{"elapsed": 2, "issued": 40}
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At the subsequent request - due to resource exhaustion - the server
advertises only "RateLimit-Remaining: 20".
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 20
RateLimit-Reset: 56
{"elapsed": 4, "issued": 41}
8.1.4. Use in throttled responses
A client exhausted its quota and the server throttles it sending
"Retry-After".
In this example, the values of "Retry-After" and "RateLimit-Reset"
reference the same moment, but this is not a requirement.
The "429 Too Many Requests" HTTP status code is just used as an
example.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
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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-Reset: 5
RateLimit-Limit: 100
Ratelimit-Remaining: 0
{
"title": "Too Many Requests",
"status": 429,
"detail": "You have exceeded your quota"
}
8.2. Parameterized responses
8.2.1. 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, 100;w=60
Ratelimit-Remaining: 99
Ratelimit-Reset: 50
{"hello": "world"}
8.2.2. Dynamic limits with parameterized windows
The policy conveyed by "RateLimit-Limit" states that the server
accepts 100 quota-units per minute.
To avoid resource exhaustion, the server artificially lowers the
actual limits returned in the throttling headers.
The "RateLimit-Remaining" then advertises only 9 quota-units for the
next 50 seconds to slow down the client.
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Note that the server could have lowered even the other values in
"RateLimit-Limit": this specification does not mandate any relation
between the field values contained in subsequent responses.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10, 100;w=60
Ratelimit-Remaining: 9
Ratelimit-Reset: 50
{
"status": 200,
"detail": "Just slow down without waiting."
}
8.2.3. Dynamic limits for pushing back and slowing down
Continuing the previous example, let's say the client waits 10
seconds and performs a new request which, due to resource exhaustion,
the server rejects and pushes back, advertising "RateLimit-Remaining:
0" for the next 20 seconds.
The server advertises a smaller window with a lower limit to slow
down the client for the rest of its original window after the 20
seconds elapse.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
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HTTP/1.1 429 Too Many Requests
Content-Type: application/json
RateLimit-Limit: 0, 15;w=20
Ratelimit-Remaining: 0
Ratelimit-Reset: 20
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
8.3. 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,
ie. 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, 100;w=60
Ratelimit-Remaining: 15
Ratelimit-Reset: 40
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
Note that in this last response the client is expected to honor
"Retry-After" and perform no requests for the specified amount of
time, whereas the previous example would not force the client to stop
requests before the reset time is elapsed, as it would still be free
to query again the server even if it is likely to have the request
rejected.
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8.3.1. Missing Remaining information
The server does not expose "RateLimit-Remaining" values, but resets
the limit counter every second.
It communicates to the client the limit of 10 quota-units per second
always returning the couple "RateLimit-Limit" and "RateLimit-Reset".
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"first": "request"}
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"second": "request"}
8.3.2. Use with multiple windows
This is a standardized way of describing the policy detailed in
Section 8.1.2:
* 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" fields to inform the client
that:
* it has only 100 quota-units left;
* the window will reset in 10 hours;
* the "expiring-limit" is 5000.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 OK
Content-Type: application/json
RateLimit-Limit: 5000, 1000;w=3600, 5000;w=86400
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"}
9. Security Considerations
9.1. Throttling does not prevent clients from issuing requests
This specification does not prevent clients to make over-quota
requests.
Servers should always implement mechanisms to prevent resource
exhaustion.
9.2. Information disclosure
Servers should not disclose operational capacity information that can
be used to saturate its resources.
While this specification does not mandate whether non 2xx responses
consume quota, if 401 and 403 responses count on quota a malicious
client could probe the endpoint to get traffic information of another
user.
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As intermediaries might retransmit requests and consume quota-units
without prior knowledge of the User Agent, RateLimit fields might
reveal the existence of an intermediary to the User Agent.
9.3. Remaining quota-units are not granted requests
"RateLimit-*" fields convey hints from the server to the clients in
order to avoid being throttled out.
Clients MUST NOT consider the "quota-units" returned in "RateLimit-
Remaining" as a service level agreement.
In case of resource saturation, the server MAY artificially lower the
returned values or not serve the request anyway.
9.4. Reliability of RateLimit-Reset
Consider that "service-limit" may not be restored after the moment
referenced by "RateLimit-Reset", and the "RateLimit-Reset" value
should not be considered fixed nor constant.
Subsequent requests may return an higher "RateLimit-Reset" value to
limit concurrency or implement dynamic or adaptive throttling
policies.
9.5. Resource exhaustion
When returning "RateLimit-Reset" you must be aware that many
throttled clients may come back at the very moment specified.
This is true for "Retry-After" too.
For example, if the quota resets every day at "18:00:00" and your
server returns the "RateLimit-Reset" accordingly
Date: Tue, 15 Nov 1994 08:00:00 GMT
RateLimit-Reset: 36000
there's a high probability that all clients will show up at
"18:00:00".
This could be mitigated by adding some jitter to the field-value.
Resource exhaustion issues can be associated with quota policies
using a large "time-window", because a user agent by chance or
purpose might consume most of its quota-units in a significantly
shorter interval.
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This behavior can be even triggered by the provided "RateLimit"
fields. The following example describes a service with an unconsumed
quota-policy of 10000 quota-units per 1000 seconds.
RateLimit-Limit: 10000, 10000;w=1000
RateLimit-Remaining: 10000
RateLimit-Reset: 10
A client implementing a simple ratio between "RateLimit-Remaining"
and "RateLimit-Reset" could infer an average throughput of 1000
quota-units per second, while "RateLimit-Limit" conveys a quota-
policy with an average of 10 quota-units per second. If the service
cannot handle such load, it should return either a lower "RateLimit-
Remaining" value or an higher "RateLimit-Reset" value. Moreover,
complementing large "time-window" quota-policies with a short "time-
window" one mitigates those risks.
9.6. Denial of Service
"RateLimit" fields may assume unexpected values by chance or purpose.
For example, an excessively high "RateLimit-Remaining" value may be:
* used by a malicious intermediary to trigger a Denial of Service
attack or consume client resources boosting its requests;
* passed by a misconfigured server;
or an high "RateLimit-Reset" value could inhibit clients to contact
the server.
Clients MUST validate the received values to mitigate those risks.
10. IANA Considerations
10.1. RateLimit-Limit Field Registration
This section registers the "RateLimit-Limit" field in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" registry ([SEMANTICS]).
Field name: "RateLimit-Limit"
Status: permanent
Specification document(s): Section 3.1 of this document
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10.2. RateLimit-Remaining Field Registration
This section registers the "RateLimit-Remaining" field in the
"Hypertext Transfer Protocol (HTTP) Field Name Registry" registry
([SEMANTICS]).
Field name: "RateLimit-Remaining"
Status: permanent
Specification document(s): Section 3.2 of this document
10.3. RateLimit-Reset Field Registration
This section registers the "RateLimit-Reset" field in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry" registry ([SEMANTICS]).
Field name: "RateLimit-Reset"
Status: permanent
Specification document(s): Section 3.3 of this document
11. References
11.1. Normative References
[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/info/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/info/rfc5234>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[SEMANTICS]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-15, 30 March 2021,
<https://www.ietf.org/archive/id/draft-ietf-httpbis-
semantics-15.txt>.
11.2. 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/info/rfc7541>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/info/rfc6585>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[STATUS429]
Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, DOI 10.17487/RFC6525, February 2012,
<https://www.rfc-editor.org/info/rfc6525>.
[UNIX] The Open Group, ., "The Single UNIX Specification, Version
2 - 6 Vol Set for UNIX 98", February 1997.
Appendix A. 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 implementors: Aapo
Talvensaari, Nathan Friedly and Sanyam Dogra.
Appendix B. FAQ
1. Why defining standard fields for throttling?
To simplify enforcement of throttling policies.
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2. Can I use RateLimit-* in throttled responses (eg with status code
429)?
Yes, you can.
3. Are those specs tied to RFC 6585?
No. [RFC6585] defines the "429" status code and we use it just
as an example of a throttled request, that could instead use even
"403" or whatever status code. The goal of this specification is
to standardize the name and semantic of three ratelimit fields
widely used on the internet. Stricter relations with status
codes or error response payloads would impose behaviors to all
the existing implementations making the adoption more complex.
4. Why don't pass the throttling scope as a parameter?
After a discussion on a similar thread
(https://github.com/httpwg/http-core/pull/317#issuecomment-
585868767) we will probably add a new "RateLimit-Scope" field to
this spec.
I'm 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 [SEMANTICS]). This may be problematic (eg.
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;
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* response-time latency may not make it worth. A brief
discussion on the subject is on the httpwg ml
(https://lists.w3.org/Archives/Public/ietf-http-
wg/2019JulSep/0202.html)
* almost all rate-limit headers implementations do not use it.
6. Why not support multiple quota remaining?
While this might be of some value, my experience suggests that
overly-complex quota implementations results in lower
effectiveness of this policy. This spec allows the client to
easily focusing on RateLimit-Remaining and RateLimit-Reset.
7. Shouldn't I limit concurrency instead of request rate?
You can use this specification to limit concurrency at the HTTP
level (see {#use-for-limiting-concurrency}) and help clients to
shape their requests avoiding being throttled out.
A problematic way to limit concurrency is connection dropping,
especially when connections are multiplexed (eg. 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 (eg. thrashing, connection
queues too long, Service Level Objectives not meet, ..).
Saturation conditions can be either dynamic or static: all this
is out of the scope for the current document.
8. Do a positive value of "RateLimit-Remaining" imply any service
guarantee for my future requests to be served?
No. FAQ integrated in Section 3.2.
9. Is the quota-policy definition Section 2.3 too complex?
You can always return the simplest form of the 3 fields
RateLimit-Limit: 100
RateLimit-Remaining: 50
RateLimit-Reset: 60
The key runtime value is the first element of the list: "expiring-
limit", the others "quota-policy" are informative. So for the
following field:
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RateLimit-Limit: 100, 100;w=60;burst=1000;comment="sliding window", 5000;w=3600;burst=0;comment="fixed window"
the key value is the one referencing the lowest limit: "100"
1. Can we use shorter names? Why don't put everything in one field?
The most common syntax we found on the web is "X-RateLimit-*" and
when starting this I-D we opted for it
(https://github.com/ioggstream/draft-polli-ratelimit-headers/
issues/34#issuecomment-519366481)
The basic form of those fields is easily parseable, even by
implementors procesing responses using technologies like dynamic
interpreter with limited syntax.
Using a single field complicates parsing and takes a significantly
different approach from the existing ones: this can limit adoption.
1. Why don't mention connections?
Beware of the term "connection":   - it is just
_one_ possible saturation cause. Once you go that path 
you will expose other infrastructural details (bandwidth, CPU, ..
see Section 9.2)  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 9.1).
  RateLimit fields enable sending _on the same
connection_ different limit values  on each response,
depending on the policy scope (eg. per-user, per-custom-key, ..)

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