| Network Working Group | R. Polli |
| Internet-Draft | Team Digitale, Italian Government |
| Intended status: Standards Track | A. Martinez |
| Expires: April 23, 2020 | Red Hat |
| October 21, 2019 |
RateLimit Header Fields for HTTP
draft-polli-ratelimit-headers-01
This document defines the RateLimit-Limit, RateLimit-Remaining, RateLimit-Reset header fields for HTTP, thus allowing servers to publish current request quotas and clients to shape their request policy and avoid being throttled out.
RFC EDITOR: please remove this section before publication
Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.
The source code and issues list for this draft can be found at https://github.com/ioggstream/draft-polli-ratelimit-headers.
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The widespreading of HTTP as a distributed computation protocol requires an explicit way of communicating service status and usage quotas.
This was partially addressed with the Retry-After header field defined in [RFC7231] to be returned in 429 Too Many Requests 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.
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.
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:
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.
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:
A major interoperability issue in throttling is the lack of standard headers, because:
Client applications 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.
This proposal defines syntax and semantics for the following header fields:
The behavior of RateLimit-Reset is compatible with the delta-seconds notation of Retry-After.
The header fields definition allows to describe complex policies, including the ones using multiple and variable time windows and dynamic quotas, or implementing concurrency limits.
The goals of this proposal are:
The goals do not include:
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] and [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 7 of [RFC7230].
The term Origin is to be interpreted as described in [RFC6454] section 7.
The “delta-seconds” rule is defined in [RFC7234] section 1.2.1.
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 = delta-seconds
Subsecond precision is not supported.
The request-quota 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 request-quota is expressed in quota-units and has the following syntax:
request-quota = quota-units quota-units = 1*DIGIT
The request-quota SHOULD match the maximum number of acceptable requests.
The request-quota MAY differ from the total number of acceptable requests when weight mechanisms, bursts, or other server policies are implemented.
If the request-quota does not match the maximum number of acceptable requests the relation with that SHOULD be communicated out-of-band.
Example: A server could
so that we have the following counters
GET /books/123 ; request-quota=4, remaining: 3, status=200 GET /books?author=Camilleri ; request-quota=4, remaining: 1, status=200 GET /books?author=Eco ; request-quota=4, remaining: 0, status=429
This specification allows describing a quota policy with the following syntax:
quota-policy = request-quota; "w" "=" time-window
*( OWS ";" OWS quota-comment)
quota-comment = token "=" (token / quoted-string)
An example policy of 100 quota-units per minute.
100;w=60
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"
The following RateLimit response header fields are defined
The RateLimit-Limit response header field indicates the request-quota associated to the client in the current time-window.
If the client exceeds that limit, it MAY not be served.
The header value is
RateLimit-Limit = expiring-limit [, 1#quota-policy ] expiring-limit = request-quota
The expiring-limit value MUST be set to the request-quota 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:
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
The RateLimit-Remaining response header field indicates the remaining quota-units defined in Section 2.2 associated to the client.
The header value is
RateLimit-Remaining = quota-units
Clients MUST NOT assume that a positive RateLimit-Remaining value is a guarantee of being served.
A low RateLimit-Remaining value is like a yellow traffic-light: the red light may arrive suddenly.
One example of RateLimit-Remaining use is below.
RateLimit-Remaining: 50
The RateLimit-Reset response header field indicates either
The header value is
RateLimit-Reset = delta-seconds
The delta-seconds format is used because:
An example of RateLimit-Reset use is below.
RateLimit-Reset: 50
The client MUST NOT assume that all its request-quota 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.
A server MAY use one or more RateLimit response header 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 header 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 header fields independently of the response status code. This includes throttled responses.
If a response contains both the Retry-After and the RateLimit-Reset header fields, the value of RateLimit-Reset MUST be consistent with the one of Retry-After.
When using a policy involving more than one time-window, the server MUST reply with the RateLimit headers related to the window with the lower RateLimit-Remaining values.
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.
A client MUST process the received RateLimit headers.
A client MUST validate the values received in the RateLimit headers 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 request-quota too high.
Malformed RateLimit headers 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 informations (eg. the Date response header, 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 header fields, the Retry-After header field MUST take precedence and the RateLimit-Reset header field MAY be ignored.
The client exhausted its request-quota for the next 50 seconds. The time-window is communicated out-of-band or inferred by the header values.
Request:
GET /items/123
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
Ratelimit-Remaining: 0
Ratelimit-Reset: 50
{"hello": "world"}
The server uses two custom headers, namely acme-RateLimit-DayLimit and acme-RateLimit-HourLimit to expose the following policy:
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-* headers to inform the client that:
Request:
GET /items/123
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"}
Throttling headers 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
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 60
RateLimit-Reset: 58
{"elapsed": 2, "issued": 40}
At the subsequent request - due to resource exhaustion - the server advertises only RateLimit-Remaining: 20.
Request:
GET /items/123
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 20
RateLimit-Reset: 56
{"elapsed": 4, "issued": 41}
A client exhausted its quota and the server throttles the request sending the Retry-After response header field.
The values of Retry-After and RateLimit-Reset are consistent as they reference the same moment.
The 429 Too Many Requests HTTP status code is just used as an example.
Request:
GET /items/123
Response:
HTTP/1.1 429 Too Many Requests
Content-Type: application/json
Date: Mon, 05 Aug 2019 09:27:00 GMT
Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
RateLimit-Reset: 5
RateLimit-Limit: 100
Ratelimit-Remaining: 0
{
"title": "Too Many Requests",
"status": 429,
"detail": "You have exceeded your quota"
}
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
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100, 100;w=60
Ratelimit-Remaining: 99
Ratelimit-Reset: 50
{"hello": "world"}
The policy conveyed by RateLimit-Limit states that the server accepts 100 quota-units per minute.
Due to resource exhaustion, the server artificially lowers the actual limits returned in the throttling headers.
The current policy then advertises only 9 quota-units for the next 50 seconds.
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
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10, 100;w=60
Ratelimit-Remaining: 9
Ratelimit-Reset: 50
{"hello": "world"}
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
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"first": "request"}
Request:
GET /items/123
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"second": "request"}
This is a standardized way of describing the policy detailed in Section 6.1.2:
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 headers to inform the client that:
Request:
GET /items/123
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"}
This specification does not prevent clients to make over-quota requests.
Servers should always implement mechanisms to prevent resource exhaustion.
Servers should not disclose operational capacity informations 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 informations of another user.
RateLimit-* headers 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.
Consider that request-quota 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.
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 adding some jitter to the field-value.
RateLimit header fields may assume unexpected values by chance or purpose. For example, an excessively high RateLimit-Remaining value may be:
or an high RateLimit-Reset value could inhibit clients to contact the server.
Clients MUST validate the received values to mitigate those risks.
This section registers the RateLimit-Limit header field in the “Permanent Message Header Field Names” registry ([RFC3864]).
Header field name: RateLimit-Limit
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 3.1 of this document
This section registers the RateLimit-Remaining header field in the “Permanent Message Header Field Names” registry ([RFC3864]).
Header field name: RateLimit-Remaining
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 3.2 of this document
This section registers the RateLimit-Reset header field in the “Permanent Message Header Field Names” registry ([RFC3864]).
Header field name: RateLimit-Reset
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document(s): Section 3.3 of this document
| [RFC3339] | Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002. |
| [RFC6585] | Nottingham, M. and R. Fielding, "Additional HTTP Status Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012. |
RFC EDITOR PLEASE DELETE THIS SECTION.
Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro Ranellucci, Erik Wilde and Mark Nottingham for being the initial contributors of these specifications.
RFC EDITOR PLEASE DELETE THIS SECTION.
Commonly used header field names are:
There are variants too, where the window is specified in the header field name, eg:
Here are some interoperability issues:
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.
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 header:
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