HTTP R. Polli Internet-Draft Team Digitale, Italian Government Intended status: Standards Track A. Martinez Expires: 4 September 2020 Red Hat 3 March 2020 RateLimit Header Fields for HTTP draft-polli-ratelimit-headers-02 Abstract 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. Note to Readers _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/ (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 (https://github.com/ioggstream/draft-polli-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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 4 September 2020. Polli & Martinez Expires 4 September 2020 [Page 1] Internet-Draft RateLimit Header Fields for HTTP March 2020 Copyright Notice Copyright (c) 2020 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 as described in Section 4.e of the Trust Legal Provisions and are 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. Request quota . . . . . . . . . . . . . . . . . . . . . . 6 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 headers . . . . . . . . . . . . . . . . . 10 5. Receiving RateLimit headers . . . . . . . . . . . . . . . . . 10 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1. Unparameterized responses . . . . . . . . . . . . . . . . 11 6.1.1. Throttling informations in responses . . . . . . . . 11 6.1.2. Use in conjunction with custom headers . . . . . . . 12 6.1.3. Use for limiting concurrency . . . . . . . . . . . . 12 6.1.4. Use in throttled responses . . . . . . . . . . . . . 13 6.2. Parameterized responses . . . . . . . . . . . . . . . . . 14 6.2.1. Throttling window specified via parameter . . . . . . 14 6.2.2. Dynamic limits with parameterized windows . . . . . . 14 6.2.3. Dynamic limits for pushing back and slowing down . . 15 6.3. Dynamic limits for pushing back with Retry-After and slow down . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.3.1. Missing Remaining informations . . . . . . . . . . . 16 6.3.2. Use with multiple windows . . . . . . . . . . . . . . 17 7. Security Considerations . . . . . . . . . . . . . . . . . . . 18 Polli & Martinez Expires 4 September 2020 [Page 2] Internet-Draft RateLimit Header Fields for HTTP March 2020 7.1. Throttling does not prevent clients from issuing requests . . . . . . . . . . . . . . . . . . . . . . . . 18 7.2. Information disclosure . . . . . . . . . . . . . . . . . 18 7.3. Remaining quota-units are not granted requests . . . . . 18 7.4. Reliability of RateLimit-Reset . . . . . . . . . . . . . 18 7.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 19 7.6. Denial of Service . . . . . . . . . . . . . . . . . . . . 19 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 8.1. RateLimit-Limit Header Field Registration . . . . . . . . 19 8.2. RateLimit-Remaining Header Field Registration . . . . . . 20 8.3. RateLimit-Reset Header Field Registration . . . . . . . . 20 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 9.1. Normative References . . . . . . . . . . . . . . . . . . 20 9.2. Informative References . . . . . . . . . . . . . . . . . 21 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 22 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 22 Appendix C. RateLimit headers currently used on the web . . . . 22 Appendix D. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 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 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. 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. 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. Polli & Martinez Expires 4 September 2020 [Page 3] Internet-Draft RateLimit Header Fields for HTTP March 2020 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. Client applications interfacing with different servers may thus need to process different headers, or the very same application interface Polli & Martinez Expires 4 September 2020 [Page 4] Internet-Draft RateLimit Header Fields for HTTP March 2020 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 header 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 "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. 1.4. Goals The goals of this proposal are: 1. Standardizing the names and semantic of rate-limit headers; 2. Improve resiliency of HTTP infrastructures simplifying the enforcement and the adoption of rate-limit headers; 3. Simplify API documentation avoiding expliciting rate-limit header fields semantic in documentation. The goals do not include: Authorization: The rate-limit headers 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 [RFC6454] section 7. Response status code: The rate-limit headers may be returned in both Successful and non Successful responses. This specification does not cover whether non Successful responses count on quota usage. Polli & Martinez Expires 4 September 2020 [Page 5] Internet-Draft RateLimit Header Fields for HTTP March 2020 Throttling policy: This specification does not mandate a specific throttling policy. The values published in the headers, 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] 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. 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 = delta-seconds Subsecond precision is not supported. 2.2. Request quota 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: Polli & Martinez Expires 4 September 2020 [Page 6] Internet-Draft RateLimit Header Fields for HTTP March 2020 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 * count once requests like "/books/{id}" * count twice search requests like "/books?author=Camilleri" 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 2.3. Quota policy 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) quota-policy parameters like "w" and quota-comment tokens MUST NOT occur multiple times within the same quota-policy. 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" Polli & Martinez Expires 4 September 2020 [Page 7] Internet-Draft RateLimit Header Fields for HTTP March 2020 3. Header Specifications The following "RateLimit" response header fields are defined 3.1. RateLimit-Limit 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: * 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 header MUST NOT occur multiple times. Polli & Martinez Expires 4 September 2020 [Page 8] Internet-Draft RateLimit Header Fields for HTTP March 2020 3.2. RateLimit-Remaining 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 This header MUST NOT occur multiple times. 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 3.3. RateLimit-Reset The "RateLimit-Reset" response header field indicates either * the number of seconds until the quota resets. The header value is RateLimit-Reset = delta-seconds The delta-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 [RFC7231] Section 4.1.1.1); * it mitigates the risk related to thundering herd when too many clients are serviced with the same timestamp. This header MUST NOT occur multiple times. An example of "RateLimit-Reset" use is below. RateLimit-Reset: 50 Polli & Martinez Expires 4 September 2020 [Page 9] Internet-Draft RateLimit Header Fields for HTTP March 2020 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. 4. Providing RateLimit headers 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" 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" 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. 5. Receiving RateLimit headers 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. Polli & Martinez Expires 4 September 2020 [Page 10] Internet-Draft RateLimit Header Fields for HTTP March 2020 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. 6. Examples 6.1. Unparameterized responses 6.1.1. Throttling informations in responses 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"} Polli & Martinez Expires 4 September 2020 [Page 11] Internet-Draft RateLimit Header Fields for HTTP March 2020 6.1.2. Use in conjunction with custom headers The server uses two custom headers, 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-*" headers to inform the client that: * it has only 100 quota-units left; * the window will reset in 10 hours. 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"} 6.1.3. Use for limiting concurrency 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 Polli & Martinez Expires 4 September 2020 [Page 12] Internet-Draft RateLimit Header Fields for HTTP March 2020 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} 6.1.4. Use in throttled responses A client exhausted its quota and the server throttles the request sending the "Retry-After" response header field. 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 Response: Polli & Martinez Expires 4 September 2020 [Page 13] Internet-Draft RateLimit Header Fields for HTTP March 2020 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" } 6.2. Parameterized responses 6.2.1. 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 Response: HTTP/1.1 200 Ok Content-Type: application/json RateLimit-Limit: 100, 100;w=60 Ratelimit-Remaining: 99 Ratelimit-Reset: 50 {"hello": "world"} 6.2.2. 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. Polli & Martinez Expires 4 September 2020 [Page 14] Internet-Draft RateLimit Header Fields for HTTP March 2020 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 { "status": 200, "detail": "Just slow down without waiting." } 6.2.3. 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 Response: 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!" } Polli & Martinez Expires 4 September 2020 [Page 15] Internet-Draft RateLimit Header Fields for HTTP March 2020 6.3. 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 the Retry-After header, 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 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 the "Retry-After" header 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. 6.3.1. Missing Remaining informations 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 Polli & Martinez Expires 4 September 2020 [Page 16] Internet-Draft RateLimit Header Fields for HTTP March 2020 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"} 6.3.2. Use with multiple windows This is a standardized way of describing the policy detailed in Section 6.1.2: * 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" headers 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 Polli & Martinez Expires 4 September 2020 [Page 17] Internet-Draft RateLimit Header Fields for HTTP March 2020 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"} 7. Security Considerations 7.1. 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. 7.2. Information disclosure 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. 7.3. Remaining quota-units are not granted requests "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. 7.4. Reliability of RateLimit-Reset 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. Polli & Martinez Expires 4 September 2020 [Page 18] Internet-Draft RateLimit Header Fields for HTTP March 2020 Subsequent requests may return an higher "RateLimit-Reset" value to limit concurrency or implement dynamic or adaptive throttling policies. 7.5. 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 adding some jitter to the field-value. 7.6. Denial of Service "RateLimit" header 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. 8. IANA Considerations 8.1. RateLimit-Limit Header Field Registration 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 Polli & Martinez Expires 4 September 2020 [Page 19] Internet-Draft RateLimit Header Fields for HTTP March 2020 Status: standard Author/Change controller: IETF Specification document(s): Section 3.1 of this document 8.2. RateLimit-Remaining Header Field Registration 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 8.3. RateLimit-Reset Header Field Registration 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 9. References 9.1. 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, . [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, Polli & Martinez Expires 4 September 2020 [Page 20] Internet-Draft RateLimit Header Fields for HTTP March 2020 DOI 10.17487/RFC3864, September 2004, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011, . [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, . [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . [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, . [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [UNIX] The Open Group, ., "The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98", February 1997. 9.2. Informative References [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, . Polli & Martinez Expires 4 September 2020 [Page 21] Internet-Draft RateLimit Header Fields for HTTP March 2020 Appendix A. Change Log RFC EDITOR PLEASE DELETE THIS SECTION. Appendix B. 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 C. RateLimit headers currently used on the web RFC EDITOR PLEASE DELETE THIS SECTION. 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 - a datetime, either "IMF-fixdate" [RFC7231] or [RFC3339] * different headers, with the same semantic, are used by different implementers: Polli & Martinez Expires 4 September 2020 [Page 22] Internet-Draft RateLimit Header Fields for HTTP March 2020 - 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-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. Appendix D. FAQ 1. Why defining standard headers for throttling? To simplify enforcement of throttling policies. 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. 4. Why don't pass the trottling scope as a parameter? I'm open to suggestions. File an issue if you think it's worth ;). 5. Why using delta-seconds instead of a UNIX Timestamp? Why not using subsecond precision? Polli & Martinez Expires 4 September 2020 [Page 23] Internet-Draft RateLimit Header Fields for HTTP March 2020 Using delta-seconds aligns with "Retry-After", which is returned in similar contexts, eg on 429 responses. delta-seconds as defined in [RFC7234] section 1.2.1 clarifies some parsing rules too. Timestamps require a clock synchronization protocol (see [RFC7231] section 4.1.1.1). 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; * 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 do both. The goal of this spec is to provide guidance for clients in shaping their requests without being throttled out. Limiting concurrency results in unserviced client requests, which is something we want to avoid. A standard 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, ..). Polli & Martinez Expires 4 September 2020 [Page 24] Internet-Draft RateLimit Header Fields for HTTP March 2020 Availability can be improved by dynamically lowering the values returned by the "RateLimit-*" headers to slow down clients, and "Retry-After" can be used to push them back. 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. The returned values were used to decide whether to serve or not _the current request_ and do not imply any guarantee that future requests will be successful. Instead they help to understand when future requests will probably be throttled. A low value for "RateLimit-Remaining" should be interpreted as a yellow traffic-light for either the number of requests issued in the "time-window" or the request throughput. 9. Is the quota-policy definition Section 2.3 too complex? You can always return the simplest form of the 3 headers 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" 1. Can we use shorter names? Why don't put everything in one header? 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 headers is easily parseable, even by implementors procesing responses using technologies like dynamic interpreter with limited syntax. Polli & Martinez Expires 4 September 2020 [Page 25] Internet-Draft RateLimit Header Fields for HTTP March 2020 Using a single header complicates parsing and takes a significantly different approach from the existing ones: this can limit adoption. Authors' Addresses Roberto Polli Team Digitale, Italian Government Email: robipolli@gmail.com Alejandro Martinez Ruiz Red Hat Email: amr@redhat.com Polli & Martinez Expires 4 September 2020 [Page 26]