draft-ietf-httpapi-ratelimit-headers-00.unpg.txt		draft-ietf-httpapi-ratelimit-headers-latest.txt
	HTTPAPI Working Group R. Polli		HTTPAPI Working Group R. Polli
	Internet-Draft Team Digitale, Italian Government		Internet-Draft Team Digitale, Italian Government
	Intended status: Standards Track A. Martinez		Intended status: Standards Track A. Martinez
	Expires: June 21, 2021 Red Hat		Expires: September 19, 2021 Red Hat
	December 18, 2020		March 18, 2021
	RateLimit Header Fields for HTTP		RateLimit Header Fields for HTTP
	draft-ietf-httpapi-ratelimit-headers-00		draft-ietf-httpapi-ratelimit-headers-latest
	Abstract		Abstract
	This document defines the RateLimit-Limit, RateLimit-Remaining,		This document defines the RateLimit-Limit, RateLimit-Remaining,
	RateLimit-Reset fields for HTTP, thus allowing servers to publish		RateLimit-Reset fields for HTTP, thus allowing servers to publish
	current request quotas and clients to shape their request policy and		current service limits and clients to shape their request policy and
	avoid being throttled out.		avoid being throttled out.
	Note to Readers		Note to Readers
	_RFC EDITOR: please remove this section before publication_		_RFC EDITOR: please remove this section before publication_
	Discussion of this draft takes place on the HTTP working group		Discussion of this draft takes place on the HTTP working group
	mailing list (httpapi@ietf.org), which is archived at		mailing list (httpapi@ietf.org), which is archived at
	https://lists.w3.org/Archives/Public/ietf-httpapi-wg/ [1].		https://lists.w3.org/Archives/Public/ietf-httpapi-wg/ [1].
	skipping to change at line 44 ¶		skipping to change at page 1, line 45 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on June 21, 2021.		This Internet-Draft will expire on September 19, 2021.
	Copyright Notice		Copyright Notice
	Copyright (c) 2020 IETF Trust and the persons identified as the		Copyright (c) 2021 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Rate-limiting and quotas		1.1. Rate-limiting and quotas . . . . . . . . . . . . . . . . 3
	1.2. Current landscape of rate-limiting headers		1.2. Current landscape of rate-limiting headers . . . . . . . 4
	1.2.1. Interoperability issues		1.2.1. Interoperability issues . . . . . . . . . . . . . . . 4
	1.3. This proposal		1.3. This proposal . . . . . . . . . . . . . . . . . . . . . . 5
	1.4. Goals		1.4. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
	1.5. Notational Conventions		1.5. Notational Conventions . . . . . . . . . . . . . . . . . 6
	2. Expressing rate-limit policies		2. Expressing rate-limit policies . . . . . . . . . . . . . . . 6
	2.1. Time window		2.1. Time window . . . . . . . . . . . . . . . . . . . . . . . 6
	2.2. Request quota		2.2. Service limit . . . . . . . . . . . . . . . . . . . . . . 6
	2.3. Quota policy		2.3. Quota policy . . . . . . . . . . . . . . . . . . . . . . 7
	3. Header Specifications		3. Header Specifications . . . . . . . . . . . . . . . . . . . . 8
	3.1. RateLimit-Limit		3.1. RateLimit-Limit . . . . . . . . . . . . . . . . . . . . . 8
	3.2. RateLimit-Remaining		3.2. RateLimit-Remaining . . . . . . . . . . . . . . . . . . . 9
	3.3. RateLimit-Reset		3.3. RateLimit-Reset . . . . . . . . . . . . . . . . . . . . . 9
	4. Providing RateLimit headers		4. Providing RateLimit fields . . . . . . . . . . . . . . . . . 10
	5. Intermediaries		5. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . 11
	6. Caching		6. Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
	7. Receiving RateLimit headers		7. Receiving RateLimit fields . . . . . . . . . . . . . . . . . 11
	8. Examples		8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
	8.1. Unparameterized responses		8.1. Unparameterized responses . . . . . . . . . . . . . . . . 12
	8.1.1. Throttling informations in responses		8.1.1. Throttling information in responses . . . . . . . . . 12
	8.1.2. Use in conjunction with custom headers		8.1.2. Use in conjunction with custom fields . . . . . . . . 13
	8.1.3. Use for limiting concurrency		8.1.3. Use for limiting concurrency . . . . . . . . . . . . 14
	8.1.4. Use in throttled responses		8.1.4. Use in throttled responses . . . . . . . . . . . . . 15
	8.2. Parameterized responses		8.2. Parameterized responses . . . . . . . . . . . . . . . . . 15
	8.2.1. Throttling window specified via parameter		8.2.1. Throttling window specified via parameter . . . . . . 15
	8.2.2. Dynamic limits with parameterized windows		8.2.2. Dynamic limits with parameterized windows . . . . . . 16
	8.2.3. Dynamic limits for pushing back and slowing down		8.2.3. Dynamic limits for pushing back and slowing down . . 16
	8.3. Dynamic limits for pushing back with Retry-After and slow		8.3. Dynamic limits for pushing back with Retry-After and slow
	down		down . . . . . . . . . . . . . . . . . . . . . . . . . . 17
	8.3.1. Missing Remaining informations
	8.3.2. Use with multiple windows		8.3.1. Missing Remaining information . . . . . . . . . . . . 18
	9. Security Considerations		8.3.2. Use with multiple windows . . . . . . . . . . . . . . 19
	9.1. Throttling does not prevent clients from issuing requests		9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
	9.2. Information disclosure		9.1. Throttling does not prevent clients from issuing requests 20
	9.3. Remaining quota-units are not granted requests		9.2. Information disclosure . . . . . . . . . . . . . . . . . 20
	9.4. Reliability of RateLimit-Reset		9.3. Remaining quota-units are not granted requests . . . . . 20
	9.5. Resource exhaustion		9.4. Reliability of RateLimit-Reset . . . . . . . . . . . . . 20
	9.6. Denial of Service		9.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 21
	10. IANA Considerations		9.6. Denial of Service . . . . . . . . . . . . . . . . . . . . 21
	10.1. RateLimit-Limit Field Registration		10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
	10.2. RateLimit-Remaining Field Registration		10.1. RateLimit-Limit Field Registration . . . . . . . . . . . 21
	10.3. RateLimit-Reset Field Registration		10.2. RateLimit-Remaining Field Registration . . . . . . . . . 22
	11. References		10.3. RateLimit-Reset Field Registration . . . . . . . . . . . 22
	11.1. Normative References		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
	11.2. Informative References		11.1. Normative References . . . . . . . . . . . . . . . . . . 22
	11.3. URIs		11.2. Informative References . . . . . . . . . . . . . . . . . 23
	Appendix A. Change Log		11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 23
	Appendix B. Acknowledgements		Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 24
	Appendix C. RateLimit headers currently used on the web		Appendix B. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 24
	Appendix D. FAQ		RateLimit fields currently used on the web . . . . . . . . . . . 27
	Authors' Addresses		Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
			D.1. Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . 28
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
	1. Introduction		1. Introduction
	The widespreading of HTTP as a distributed computation protocol		The widespreading of HTTP as a distributed computation protocol
	requires an explicit way of communicating service status and usage		requires an explicit way of communicating service status and usage
	quotas.		quotas.
	This was partially addressed with the "Retry-After" header field		This was partially addressed by the "Retry-After" header field
	defined in [SEMANTICS] to be returned in "429 Too Many Requests" or		defined in [SEMANTICS] to be returned in "429 Too Many Requests" (see
	"503 Service Unavailable" responses.		[STATUS429]) or "503 Service Unavailable" responses.
	Still, there is not a standard way to communicate service quotas so		Still, there is not a standard way to communicate service quotas so
	that the client can throttle its requests and prevent 4xx or 5xx		that the client can throttle its requests and prevent 4xx or 5xx
	responses.		responses.
	1.1. Rate-limiting and quotas		1.1. Rate-limiting and quotas
	Servers use quota mechanisms to avoid systems overload, to ensure an		Servers use quota mechanisms to avoid systems overload, to ensure an
	equitable distribution of computational resources or to enforce other		equitable distribution of computational resources or to enforce other
	policies - eg. monetization.		policies - eg. monetization.
	skipping to change at line 189 ¶		skipping to change at page 5, line 5 ¶
	1.2.1. Interoperability issues		1.2.1. Interoperability issues
	A major interoperability issue in throttling is the lack of standard		A major interoperability issue in throttling is the lack of standard
	headers, because:		headers, because:
	o each implementation associates different semantics to the same		o each implementation associates different semantics to the same
	header field names;		header field names;
	o header field names proliferates.		o header field names proliferates.
	Client applications interfacing with different servers may thus need		User Agents interfacing with different servers may thus need to
	to process different headers, or the very same application interface		process different headers, or the very same application interface
	that sits behind different reverse proxies may reply with different		that sits behind different reverse proxies may reply with different
	throttling headers.		throttling headers.
	1.3. This proposal		1.3. This proposal
	This proposal defines syntax and semantics for the following fields:		This proposal defines syntax and semantics for the following fields:
	o "RateLimit-Limit": containing the requests quota in the time		o "RateLimit-Limit": containing the requests quota in the time
	window;		window;
	o "RateLimit-Remaining": containing the remaining requests quota in		o "RateLimit-Remaining": containing the remaining requests quota in
	the current window;		the current window;
	o "RateLimit-Reset": containing the time remaining in the current		o "RateLimit-Reset": containing the time remaining in the current
	window, specified in seconds.		window, specified in seconds.
	The behavior of "RateLimit-Reset" is compatible with the "delta-		The behavior of "RateLimit-Reset" is compatible with the "delay-
	seconds" notation of "Retry-After".		seconds" notation of "Retry-After".
	The fields definition allows to describe complex policies, including		The fields definition allows to describe complex policies, including
	the ones using multiple and variable time windows and dynamic quotas,		the ones using multiple and variable time windows and dynamic quotas,
	or implementing concurrency limits.		or implementing concurrency limits.
	1.4. Goals		1.4. Goals
	The goals of this proposal are:		The goals of this proposal are:
	1. Standardizing the names and semantic of rate-limit headers;		1. Standardizing the names and semantic of rate-limit headers;
	2. Improve resiliency of HTTP infrastructures simplifying the		2. Improve resiliency of HTTP infrastructures simplifying the
	enforcement and the adoption of rate-limit headers;		enforcement and the adoption of rate-limit headers;
	3. Simplify API documentation avoiding expliciting rate-limit fields		3. Simplify API documentation avoiding expliciting rate-limit fields
	semantic in documentation.		semantic in documentation.
	The goals do not include:		The goals do not include:
	Authorization: The rate-limit headers described here are not meant		Authorization: The rate-limit fields described here are not meant to
	to support authorization or other kinds of access controls.		support authorization or other kinds of access controls.
	Throttling scope: This specification does not cover the throttling		Throttling scope: This specification does not cover the throttling
	scope, that may be the given resource-target, its parent path or		scope, that may be the given resource-target, its parent path or
	the whole Origin [RFC6454] section 7.		the whole Origin (see Section 7 of [RFC6454]).
	Response status code: The rate-limit headers may be returned in both		Response status code: The rate-limit fields may be returned in both
	Successful and non Successful responses. This specification does		Successful and non Successful responses. This specification does
	not cover whether non Successful responses count on quota usage.		not cover whether non Successful responses count on quota usage.
	Throttling policy: This specification does not mandate a specific		Throttling policy: This specification does not mandate a specific
	throttling policy. The values published in the headers, including		throttling policy. The values published in the fields, including
	the window size, can be statically or dynamically evaluated.		the window size, can be statically or dynamically evaluated.
	Service Level Agreement: Conveyed quota hints do not imply any		Service Level Agreement: Conveyed quota hints do not imply any
	service guarantee. Server is free to throttle respectful clients		service guarantee. Server is free to throttle respectful clients
	under certain circumstances.		under certain circumstances.
	1.5. Notational Conventions		1.5. Notational Conventions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in
	BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all		BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	This document uses the Augmented BNF defined in [RFC5234] and updated		This document uses the Augmented BNF defined in [RFC5234] and updated
	by [RFC7405] along with the "#rule" extension defined in Section 7 of		by [RFC7405] along with the "#rule" extension defined in
	[MESSAGING].		Section 5.6.1 of [SEMANTICS].
	The term Origin is to be interpreted as described in [RFC6454]		The term Origin is to be interpreted as described in Section 7 of
	section 7.		[RFC6454].
	The "delta-seconds" rule is defined in [CACHING] section 1.2.1.		The "delay-seconds" rule is defined in Section 10.2.4 of [SEMANTICS].
	2. Expressing rate-limit policies		2. Expressing rate-limit policies
	2.1. Time window		2.1. Time window
	Rate limit policies limit the number of acceptable requests in a		Rate limit policies limit the number of acceptable requests in a
	given time window.		given time window.
	A time window is expressed in seconds, using the following syntax:		A time window is expressed in seconds, using the following syntax:
	time-window = delta-seconds		time-window = delay-seconds
	Subsecond precision is not supported.		Subsecond precision is not supported.
	2.2. Request quota		2.2. Service limit
	The request-quota is a value associated to the maximum number of		The service-limit is a value associated to the maximum number of
	requests that the server is willing to accept from one or more		requests that the server is willing to accept from one or more
	clients on a given basis (originating IP, authenticated user,		clients on a given basis (originating IP, authenticated user,
	geographical, ..) during a "time-window" as defined in Section 2.1.		geographical, ..) during a "time-window" as defined in Section 2.1.
	The "request-quota" is expressed in "quota-units" and has the		The "service-limit" is expressed in "quota-units" and has the
	following syntax:		following syntax:
	request-quota = quota-units		service-limit = quota-units
	quota-units = 1*DIGIT		quota-units = 1*DIGIT
	The "request-quota" SHOULD match the maximum number of acceptable		The "service-limit" SHOULD match the maximum number of acceptable
	requests.		requests.
	The "request-quota" MAY differ from the total number of acceptable		The "service-limit" MAY differ from the total number of acceptable
	requests when weight mechanisms, bursts, or other server policies are		requests when weight mechanisms, bursts, or other server policies are
	implemented.		implemented.
	If the "request-quota" does not match the maximum number of		If the "service-limit" does not match the maximum number of
	acceptable requests the relation with that SHOULD be communicated		acceptable requests the relation with that SHOULD be communicated
	out-of-band.		out-of-band.
	Example: A server could		Example: A server could
	o count once requests like "/books/{id}"		o count once requests like "/books/{id}"
	o count twice search requests like "/books?author=Camilleri"		o count twice search requests like "/books?author=Camilleri"
	so that we have the following counters		so that we have the following counters
	GET /books/123 ; request-quota=4, remaining: 3, status=200		GET /books/123 ; service-limit=4, remaining: 3, status=200
	GET /books?author=Camilleri ; request-quota=4, remaining: 1, status=200		GET /books?author=Camilleri ; service-limit=4, remaining: 1, status=200
	GET /books?author=Eco ; request-quota=4, remaining: 0, status=429		GET /books?author=Eco ; service-limit=4, remaining: 0, status=429
	2.3. Quota policy		2.3. Quota policy
	This specification allows describing a quota policy with the		This specification allows describing a quota policy with the
	following syntax:		following syntax:
	quota-policy = request-quota; "w" "=" time-window		quota-policy = service-limit; "w" "=" time-window
	*( OWS ";" OWS quota-comment)		*( OWS ";" OWS quota-comment)
	quota-comment = token "=" (token / quoted-string)		quota-comment = token "=" (token / quoted-string)
	quota-policy parameters like "w" and quota-comment tokens MUST NOT		quota-policy parameters like "w" and quota-comment tokens MUST NOT
	occur multiple times within the same quota-policy.		occur multiple times within the same quota-policy.
	An example policy of 100 quota-units per minute.		An example policy of 100 quota-units per minute.
	100;w=60		100;w=60
	skipping to change at line 341 ¶		skipping to change at page 8, line 14 ¶
	100;w=60;comment="fixed window"		100;w=60;comment="fixed window"
	12;w=1;burst=1000;policy="leaky bucket"		12;w=1;burst=1000;policy="leaky bucket"
	3. Header Specifications		3. Header Specifications
	The following "RateLimit" response fields are defined		The following "RateLimit" response fields are defined
	3.1. RateLimit-Limit		3.1. RateLimit-Limit
	The "RateLimit-Limit" response field indicates the "request-quota"		The "RateLimit-Limit" response field indicates the "service-limit"
	associated to the client in the current "time-window".		associated to the client in the current "time-window".
	If the client exceeds that limit, it MAY not be served.		If the client exceeds that limit, it MAY not be served.
	The header value is		The field value is
	RateLimit-Limit = expiring-limit [, 1#quota-policy ]		RateLimit-Limit = expiring-limit [, 1#quota-policy ]
	expiring-limit = request-quota		expiring-limit = service-limit
	The "expiring-limit" value MUST be set to the "request-quota" that is		The "expiring-limit" value MUST be set to the "service-limit" that is
	closer to reach its limit.		closer to reach its limit.
	The "quota-policy" is defined in Section 2.3, and its values are		The "quota-policy" is defined in Section 2.3, and its values are
	informative.		informative.
	RateLimit-Limit: 100		RateLimit-Limit: 100
	A "time-window" associated to "expiring-limit" can be communicated		A "time-window" associated to "expiring-limit" can be communicated
	via an optional "quota-policy" value, like shown in the following		via an optional "quota-policy" value, like shown in the following
	example		example
	skipping to change at line 379 ¶		skipping to change at page 9, line 5 ¶
	reset, or		reset, or
	o communicated out-of-band (eg. in the documentation).		o communicated out-of-band (eg. in the documentation).
	Policies using multiple quota limits MAY be returned using multiple		Policies using multiple quota limits MAY be returned using multiple
	"quota-policy" items, like shown in the following two examples:		"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, 50;w=60, 1000;w=3600, 5000;w=86400
	RateLimit-Limit: 10, 10;w=1;burst=1000, 1000;w=3600		RateLimit-Limit: 10, 10;w=1;burst=1000, 1000;w=3600
	This header MUST NOT occur multiple times and can be sent in a		This field MUST NOT occur multiple times and can be sent in a trailer
	trailer section.		section.
	3.2. RateLimit-Remaining		3.2. RateLimit-Remaining
	The "RateLimit-Remaining" response field indicates the remaining		The "RateLimit-Remaining" response field indicates the remaining
	"quota-units" defined in Section 2.2 associated to the client.		"quota-units" defined in Section 2.2 associated to the client.
	The header value is		The field value is
	RateLimit-Remaining = quota-units		RateLimit-Remaining = quota-units
	This header MUST NOT occur multiple times and can be sent in a		This field MUST NOT occur multiple times and can be sent in a trailer
	trailer section.		section.
	Clients MUST NOT assume that a positive "RateLimit-Remaining" value		Clients MUST NOT assume that a positive "RateLimit-Remaining" value
	is a guarantee of being served.		is a guarantee that further requests will be served.
	A low "RateLimit-Remaining" value is like a yellow traffic-light: the		A low "RateLimit-Remaining" value is like a yellow traffic-light for
	red light may arrive suddenly.		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.		One example of "RateLimit-Remaining" use is below.
	RateLimit-Remaining: 50		RateLimit-Remaining: 50
	3.3. RateLimit-Reset		3.3. RateLimit-Reset
	The "RateLimit-Reset" response field indicates either		The "RateLimit-Reset" response field indicates either
	o the number of seconds until the quota resets.		o the number of seconds until the quota resets.
	The header value is		The field value is
	RateLimit-Reset = delta-seconds		RateLimit-Reset = delay-seconds
	The delta-seconds format is used because:		The delay-seconds format is used because:
	o it does not rely on clock synchronization and is resilient to		o it does not rely on clock synchronization and is resilient to
	clock adjustment and clock skew between client and server (see		clock adjustment and clock skew between client and server (see
	[SEMANTICS] Section 4.1.1.1);		Section 5.6.7 of [SEMANTICS]);
	o it mitigates the risk related to thundering herd when too many		o it mitigates the risk related to thundering herd when too many
	clients are serviced with the same timestamp.		clients are serviced with the same timestamp.
	This header MUST NOT occur multiple times and can be sent in a		This field MUST NOT occur multiple times and can be sent in a trailer
	trailer section.		section.
	An example of "RateLimit-Reset" use is below.		An example of "RateLimit-Reset" use is below.
	RateLimit-Reset: 50		RateLimit-Reset: 50
	The client MUST NOT assume that all its "request-quota" will be		The client MUST NOT assume that all its "service-limit" will be
	restored after the moment referenced by "RateLimit-Reset". The		restored after the moment referenced by "RateLimit-Reset". The
	server MAY arbitrarily alter the "RateLimit-Reset" value between		server MAY arbitrarily alter the "RateLimit-Reset" value between
	subsequent requests eg. in case of resource saturation or to		subsequent requests eg. in case of resource saturation or to
	implement sliding window policies.		implement sliding window policies.
	4. Providing RateLimit headers		4. Providing RateLimit fields
	A server MAY use one or more "RateLimit" response fields defined in		A server MAY use one or more "RateLimit" response fields defined in
	this document to communicate its quota policies.		this document to communicate its quota policies.
	The returned values refers to the metrics used to evaluate if the		The returned values refers to the metrics used to evaluate if the
	current request respects the quota policy and MAY not apply to		current request respects the quota policy and MAY not apply to
	subsequent requests.		subsequent requests.
	Example: a successful response with the following fields		Example: a successful response with the following fields
	skipping to change at line 463 ¶		skipping to change at page 10, line 42 ¶
	example from Section 2.2.		example from Section 2.2.
	A server MAY return "RateLimit" response fields independently of the		A server MAY return "RateLimit" response fields independently of the
	response status code. This includes throttled responses.		response status code. This includes throttled responses.
	If a response contains both the "Retry-After" and the "RateLimit-		If a response contains both the "Retry-After" and the "RateLimit-
	Reset" fields, the value of "RateLimit-Reset" SHOULD reference the		Reset" fields, the value of "RateLimit-Reset" SHOULD reference the
	same point in time as "Retry-After".		same point in time as "Retry-After".
	When using a policy involving more than one "time-window", the server		When using a policy involving more than one "time-window", the server
	MUST reply with the "RateLimit" headers related to the window with		MUST reply with the "RateLimit" fields related to the window with the
	the lower "RateLimit-Remaining" values.		lower "RateLimit-Remaining" values.
	Under certain conditions, a server MAY artificially lower "RateLimit"		Under certain conditions, a server MAY artificially lower "RateLimit"
	field values between subsequent requests, eg. to respond to Denial of		field values between subsequent requests, eg. to respond to Denial of
	Service attacks or in case of resource saturation.		Service attacks or in case of resource saturation.
	Servers usually establish whether the request is in-quota before		Servers usually establish whether the request is in-quota before
	creating a response, so the RateLimit field values should be already		creating a response, so the RateLimit field values should be already
	available in that moment. Nonetheless servers MAY decide to send the		available in that moment. Nonetheless servers MAY decide to send the
	"RateLimit" fields in a trailer section.		"RateLimit" fields in a trailer section.
	5. Intermediaries		5. Intermediaries
	This section documents the considerations advised in Section 15.3.3		This section documents the considerations advised in Section 16.3.3
	of [SEMANTICS].		of [SEMANTICS].
	An intermediary that is not part of the originating service		An intermediary that is not part of the originating service
	infrastructure and is not aware of the quota-policy semantic used by		infrastructure and is not aware of the quota-policy semantic used by
	the Origin Server SHOULD NOT alter the RateLimit fields' values in		the Origin Server SHOULD NOT alter the RateLimit fields' values in
	such a way as to communicate a more permissive quota-policy; this		such a way as to communicate a more permissive quota-policy; this
	includes removing the RateLimit fields.		includes removing the RateLimit fields.
	An intermediary MAY alter the RateLimit fields in such a way as to		An intermediary MAY alter the RateLimit fields in such a way as to
	communicate a more restrictive quota-policy when:		communicate a more restrictive quota-policy when:
	skipping to change at line 509 ¶		skipping to change at page 11, line 39 ¶
	This specification does not mandate any behavior on intermediaries		This specification does not mandate any behavior on intermediaries
	respect to retries, nor requires that intermediaries have any role in		respect to retries, nor requires that intermediaries have any role in
	respecting quota-policies. For example, it is legitimate for a proxy		respecting quota-policies. For example, it is legitimate for a proxy
	to retransmit a request without notifying the client, and thus		to retransmit a request without notifying the client, and thus
	consuming quota-units.		consuming quota-units.
	6. Caching		6. Caching
	As is the ordinary case for HTTP caching ([RFC7234]), a response with		As is the ordinary case for HTTP caching ([RFC7234]), a response with
	RateLimit fields might be cached and re-used for subsequent requests.		RateLimit fields might be cached and re-used for subsequent requests.
	A cached RateLimit response, does not modify quota counters but could		A cached "RateLimit" response does not modify quota counters but
	contain stale information. Clients interested in determining the		could contain stale information. Clients interested in determining
	freshness of the RateLimit fields could rely on fields such as "Date"		the freshness of the "RateLimit" fields could rely on fields such as
	and on the "window" value of a "quota-policy".		"Date" and on the "time-window" of a "quota-policy".
	7. Receiving RateLimit headers		7. Receiving RateLimit fields
	A client MUST process the received "RateLimit" headers.		A client MUST process the received "RateLimit" fields.
	A client MUST validate the values received in the "RateLimit" headers		A client MUST validate the values received in the "RateLimit" fields
	before using them and check if there are significant discrepancies		before using them and check if there are significant discrepancies
	with the expected ones. This includes a "RateLimit-Reset" moment too		with the expected ones. This includes a "RateLimit-Reset" moment too
	far in the future or a "request-quota" too high.		far in the future or a "service-limit" too high.
	Malformed "RateLimit" headers MAY be ignored.		Malformed "RateLimit" fields MAY be ignored.
	A client SHOULD NOT exceed the "quota-units" expressed in "RateLimit-		A client SHOULD NOT exceed the "quota-units" expressed in "RateLimit-
	Remaining" before the "time-window" expressed in "RateLimit-Reset".		Remaining" before the "time-window" expressed in "RateLimit-Reset".
	A client MAY still probe the server if the "RateLimit-Reset" is		A client MAY still probe the server if the "RateLimit-Reset" is
	considered too high.		considered too high.
	The value of "RateLimit-Reset" is generated at response time: a		The value of "RateLimit-Reset" is generated at response time: a
	client aware of a significant network latency MAY behave accordingly		client aware of a significant network latency MAY behave accordingly
	and use other informations (eg. the "Date" response header, or		and use other information (eg. the "Date" response header field, or
	otherwise gathered metrics) to better estimate the "RateLimit-Reset"		otherwise gathered metrics) to better estimate the "RateLimit-Reset"
	moment intended by the server.		moment intended by the server.
	The "quota-policy" values and comments provided in "RateLimit-Limit"		The "quota-policy" values and comments provided in "RateLimit-Limit"
	are informative and MAY be ignored.		are informative and MAY be ignored.
	If a response contains both the "RateLimit-Reset" and "Retry-After"		If a response contains both the "RateLimit-Reset" and "Retry-After"
	fields, the "Retry-After" header field MUST take precedence and the		fields, "Retry-After" MUST take precedence and "RateLimit-Reset" MAY
	"RateLimit-Reset" field MAY be ignored.		be ignored.
			This specification does not mandate a specific throttling behavior
			and implementers can adopt their preferred policies, including:
			o slowing down or preemptively backoff their request rate when
			approaching quota limits;
			o consuming all the quota according to the exposed limits and then
			wait.
	8. Examples		8. Examples
	8.1. Unparameterized responses		8.1. Unparameterized responses
	8.1.1. Throttling informations in responses		8.1.1. Throttling information in responses
	The client exhausted its request-quota for the next 50 seconds. The		The client exhausted its service-limit for the next 50 seconds. The
	"time-window" is communicated out-of-band or inferred by the header		"time-window" is communicated out-of-band or inferred by the field
	values.		values.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 100		RateLimit-Limit: 100
	Ratelimit-Remaining: 0		Ratelimit-Remaining: 0
	Ratelimit-Reset: 50		Ratelimit-Reset: 50
	{"hello": "world"}		{"hello": "world"}
	8.1.2. Use in conjunction with custom headers		8.1.2. Use in conjunction with custom fields
	The server uses two custom headers, namely "acme-RateLimit-DayLimit"		The server uses two custom fields, namely "acme-RateLimit-DayLimit"
	and "acme-RateLimit-HourLimit" to expose the following policy:		and "acme-RateLimit-HourLimit" to expose the following policy:
	o 5000 daily quota-units;		o 5000 daily quota-units;
	o 1000 hourly quota-units.		o 1000 hourly quota-units.
	The client consumed 4900 quota-units in the first 14 hours.		The client consumed 4900 quota-units in the first 14 hours.
	Despite the next hourly limit of 1000 quota-units, the closest limit		Despite the next hourly limit of 1000 quota-units, the closest limit
	to reach is the daily one.		to reach is the daily one.
	The server then exposes the "RateLimit-*" headers to inform the		The server then exposes the "RateLimit-*" fields to inform the client
	client that:		that:
	o it has only 100 quota-units left;		o it has only 100 quota-units left;
	o the window will reset in 10 hours.		o the window will reset in 10 hours.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	acme-RateLimit-DayLimit: 5000		acme-RateLimit-DayLimit: 5000
	acme-RateLimit-HourLimit: 1000		acme-RateLimit-HourLimit: 1000
	RateLimit-Limit: 5000		RateLimit-Limit: 5000
	RateLimit-Remaining: 100		RateLimit-Remaining: 100
	RateLimit-Reset: 36000		RateLimit-Reset: 36000
	{"hello": "world"}		{"hello": "world"}
	8.1.3. Use for limiting concurrency		8.1.3. Use for limiting concurrency
	Throttling headers may be used to limit concurrency, advertising		Throttling fields may be used to limit concurrency, advertising
	limits that are lower than the usual ones in case of saturation, thus		limits that are lower than the usual ones in case of saturation, thus
	increasing availability.		increasing availability.
	The server adopted a basic policy of 100 quota-units per minute, and		The server adopted a basic policy of 100 quota-units per minute, and
	in case of resource exhaustion adapts the returned values reducing		in case of resource exhaustion adapts the returned values reducing
	both "RateLimit-Limit" and "RateLimit-Remaining".		both "RateLimit-Limit" and "RateLimit-Remaining".
	After 2 seconds the client consumed 40 quota-units		After 2 seconds the client consumed 40 quota-units
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 100		RateLimit-Limit: 100
	RateLimit-Remaining: 60		RateLimit-Remaining: 60
	RateLimit-Reset: 58		RateLimit-Reset: 58
	{"elapsed": 2, "issued": 40}		{"elapsed": 2, "issued": 40}
	At the subsequent request - due to resource exhaustion - the server		At the subsequent request - due to resource exhaustion - the server
	advertises only "RateLimit-Remaining: 20".		advertises only "RateLimit-Remaining: 20".
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 100		RateLimit-Limit: 100
	RateLimit-Remaining: 20		RateLimit-Remaining: 20
	RateLimit-Reset: 56		RateLimit-Reset: 56
	{"elapsed": 4, "issued": 41}		{"elapsed": 4, "issued": 41}
	8.1.4. Use in throttled responses		8.1.4. Use in throttled responses
	A client exhausted its quota and the server throttles the request		A client exhausted its quota and the server throttles it sending
	sending the "Retry-After" response header field.		"Retry-After".
	In this example, the values of "Retry-After" and "RateLimit-Reset"		In this example, the values of "Retry-After" and "RateLimit-Reset"
	reference the same moment, but this is not a requirement.		reference the same moment, but this is not a requirement.
	The "429 Too Many Requests" HTTP status code is just used as an		The "429 Too Many Requests" HTTP status code is just used as an
	example.		example.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 429 Too Many Requests		HTTP/1.1 429 Too Many Requests
	Content-Type: application/json		Content-Type: application/json
	Date: Mon, 05 Aug 2019 09:27:00 GMT		Date: Mon, 05 Aug 2019 09:27:00 GMT
	Retry-After: Mon, 05 Aug 2019 09:27:05 GMT		Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
	RateLimit-Reset: 5		RateLimit-Reset: 5
	RateLimit-Limit: 100		RateLimit-Limit: 100
	Ratelimit-Remaining: 0		Ratelimit-Remaining: 0
	skipping to change at line 689 ¶		skipping to change at page 15, line 47 ¶
	8.2. Parameterized responses		8.2. Parameterized responses
	8.2.1. Throttling window specified via parameter		8.2.1. Throttling window specified via parameter
	The client has 99 "quota-units" left for the next 50 seconds. The		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		"time-window" is communicated by the "w" parameter, so we know the
	throughput is 100 "quota-units" per minute.		throughput is 100 "quota-units" per minute.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 100, 100;w=60		RateLimit-Limit: 100, 100;w=60
	Ratelimit-Remaining: 99		Ratelimit-Remaining: 99
	Ratelimit-Reset: 50		Ratelimit-Reset: 50
	{"hello": "world"}		{"hello": "world"}
	skipping to change at line 718 ¶		skipping to change at page 16, line 30 ¶
	The "RateLimit-Remaining" then advertises only 9 quota-units for the		The "RateLimit-Remaining" then advertises only 9 quota-units for the
	next 50 seconds to slow down the client.		next 50 seconds to slow down the client.
	Note that the server could have lowered even the other values in		Note that the server could have lowered even the other values in
	"RateLimit-Limit": this specification does not mandate any relation		"RateLimit-Limit": this specification does not mandate any relation
	between the field values contained in subsequent responses.		between the field values contained in subsequent responses.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 10, 100;w=60		RateLimit-Limit: 10, 100;w=60
	Ratelimit-Remaining: 9		Ratelimit-Remaining: 9
	Ratelimit-Reset: 50		Ratelimit-Reset: 50
	{		{
	skipping to change at line 746 ¶		skipping to change at page 17, line 11 ¶
	seconds and performs a new request which, due to resource exhaustion,		seconds and performs a new request which, due to resource exhaustion,
	the server rejects and pushes back, advertising "RateLimit-Remaining:		the server rejects and pushes back, advertising "RateLimit-Remaining:
	0" for the next 20 seconds.		0" for the next 20 seconds.
	The server advertises a smaller window with a lower limit to slow		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		down the client for the rest of its original window after the 20
	seconds elapse.		seconds elapse.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 429 Too Many Requests		HTTP/1.1 429 Too Many Requests
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 0, 15;w=20		RateLimit-Limit: 0, 15;w=20
	Ratelimit-Remaining: 0		Ratelimit-Remaining: 0
	Ratelimit-Reset: 20		Ratelimit-Reset: 20
	{		{
	"status": 429,		"status": 429,
	"detail": "Wait 20 seconds, then slow down!"		"detail": "Wait 20 seconds, then slow down!"
	}		}
	8.3. Dynamic limits for pushing back with Retry-After and slow down		8.3. Dynamic limits for pushing back with Retry-After and slow down
	Alternatively, given the same context where the previous example		Alternatively, given the same context where the previous example
	starts, we can convey the same information to the client via the		starts, we can convey the same information to the client via "Retry-
	Retry-After header, with the advantage that the server can now		After", with the advantage that the server can now specify the
	specify the policy's nominal limit and window that will apply after		policy's nominal limit and window that will apply after the reset,
	the reset, ie. assuming the resource exhaustion is likely to be gone		ie. assuming the resource exhaustion is likely to be gone by then, so
	by then, so the advertised policy does not need to be adjusted, yet		the advertised policy does not need to be adjusted, yet we managed to
	we managed to stop requests for a while and slow down the rest of the		stop requests for a while and slow down the rest of the current
	current window.		window.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 429 Too Many Requests		HTTP/1.1 429 Too Many Requests
	Content-Type: application/json		Content-Type: application/json
	Retry-After: 20		Retry-After: 20
	RateLimit-Limit: 15, 100;w=60		RateLimit-Limit: 15, 100;w=60
	Ratelimit-Remaining: 15		Ratelimit-Remaining: 15
	Ratelimit-Reset: 40		Ratelimit-Reset: 40
	{		{
	"status": 429,		"status": 429,
	"detail": "Wait 20 seconds, then slow down!"		"detail": "Wait 20 seconds, then slow down!"
	}		}
	Note that in this last response the client is expected to honor the		Note that in this last response the client is expected to honor
	"Retry-After" header and perform no requests for the specified amount		"Retry-After" and perform no requests for the specified amount of
	of time, whereas the previous example would not force the client to		time, whereas the previous example would not force the client to stop
	stop requests before the reset time is elapsed, as it would still be		requests before the reset time is elapsed, as it would still be free
	free to query again the server even if it is likely to have the		to query again the server even if it is likely to have the request
	request rejected.		rejected.
	8.3.1. Missing Remaining informations		8.3.1. Missing Remaining information
	The server does not expose "RateLimit-Remaining" values, but resets		The server does not expose "RateLimit-Remaining" values, but resets
	the limit counter every second.		the limit counter every second.
	It communicates to the client the limit of 10 quota-units per second		It communicates to the client the limit of 10 quota-units per second
	always returning the couple "RateLimit-Limit" and "RateLimit-Reset".		always returning the couple "RateLimit-Limit" and "RateLimit-Reset".
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 10		RateLimit-Limit: 10
	Ratelimit-Reset: 1		Ratelimit-Reset: 1
	{"first": "request"}		{"first": "request"}
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 Ok		HTTP/1.1 200 Ok
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 10		RateLimit-Limit: 10
	Ratelimit-Reset: 1		Ratelimit-Reset: 1
	{"second": "request"}		{"second": "request"}
	8.3.2. Use with multiple windows		8.3.2. Use with multiple windows
	skipping to change at line 845 ¶		skipping to change at page 19, line 27 ¶
	o 5000 daily quota-units;		o 5000 daily quota-units;
	o 1000 hourly quota-units.		o 1000 hourly quota-units.
	The client consumed 4900 quota-units in the first 14 hours.		The client consumed 4900 quota-units in the first 14 hours.
	Despite the next hourly limit of 1000 quota-units, the closest limit		Despite the next hourly limit of 1000 quota-units, the closest limit
	to reach is the daily one.		to reach is the daily one.
	The server then exposes the "RateLimit" headers to inform the client		The server then exposes the "RateLimit" fields to inform the client
	that:		that:
	o it has only 100 quota-units left;		o it has only 100 quota-units left;
	o the window will reset in 10 hours;		o the window will reset in 10 hours;
	o the "expiring-limit" is 5000.		o the "expiring-limit" is 5000.
	Request:		Request:
	GET /items/123		GET /items/123 HTTP/1.1
			Host: api.example
	Response:		Response:
	HTTP/1.1 200 OK		HTTP/1.1 200 OK
	Content-Type: application/json		Content-Type: application/json
	RateLimit-Limit: 5000, 1000;w=3600, 5000;w=86400		RateLimit-Limit: 5000, 1000;w=3600, 5000;w=86400
	RateLimit-Remaining: 100		RateLimit-Remaining: 100
	RateLimit-Reset: 36000		RateLimit-Reset: 36000
	{"hello": "world"}		{"hello": "world"}
	skipping to change at line 880 ¶		skipping to change at page 20, line 17 ¶
	9.1. Throttling does not prevent clients from issuing requests		9.1. Throttling does not prevent clients from issuing requests
	This specification does not prevent clients to make over-quota		This specification does not prevent clients to make over-quota
	requests.		requests.
	Servers should always implement mechanisms to prevent resource		Servers should always implement mechanisms to prevent resource
	exhaustion.		exhaustion.
	9.2. Information disclosure		9.2. Information disclosure
	Servers should not disclose operational capacity informations that		Servers should not disclose operational capacity information that can
	can be used to saturate its resources.		be used to saturate its resources.
	While this specification does not mandate whether non 2xx responses		While this specification does not mandate whether non 2xx responses
	consume quota, if 401 and 403 responses count on quota a malicious		consume quota, if 401 and 403 responses count on quota a malicious
	client could probe the endpoint to get traffic informations of		client could probe the endpoint to get traffic information of another
	another user.		user.
	As intermediaries might retransmit requests and consume quota-units		As intermediaries might retransmit requests and consume quota-units
	without prior knowledge of the User Agent, RateLimit headers might		without prior knowledge of the User Agent, RateLimit fields might
	reveal the existence of an intermediary to the User Agent.		reveal the existence of an intermediary to the User Agent.
	9.3. Remaining quota-units are not granted requests		9.3. Remaining quota-units are not granted requests
	"RateLimit-*" headers convey hints from the server to the clients in		"RateLimit-*" fields convey hints from the server to the clients in
	order to avoid being throttled out.		order to avoid being throttled out.
	Clients MUST NOT consider the "quota-units" returned in "RateLimit-		Clients MUST NOT consider the "quota-units" returned in "RateLimit-
	Remaining" as a service level agreement.		Remaining" as a service level agreement.
	In case of resource saturation, the server MAY artificially lower the		In case of resource saturation, the server MAY artificially lower the
	returned values or not serve the request anyway.		returned values or not serve the request anyway.
	9.4. Reliability of RateLimit-Reset		9.4. Reliability of RateLimit-Reset
	Consider that "request-quota" may not be restored after the moment		Consider that "service-limit" may not be restored after the moment
	referenced by "RateLimit-Reset", and the "RateLimit-Reset" value		referenced by "RateLimit-Reset", and the "RateLimit-Reset" value
	should not be considered fixed nor constant.		should not be considered fixed nor constant.
	Subsequent requests may return an higher "RateLimit-Reset" value to		Subsequent requests may return an higher "RateLimit-Reset" value to
	limit concurrency or implement dynamic or adaptive throttling		limit concurrency or implement dynamic or adaptive throttling
	policies.		policies.
	9.5. Resource exhaustion		9.5. Resource exhaustion
	When returning "RateLimit-Reset" you must be aware that many		When returning "RateLimit-Reset" you must be aware that many
	skipping to change at line 986 ¶		skipping to change at page 22, line 32 ¶
	Field name: "RateLimit-Reset"		Field name: "RateLimit-Reset"
	Status: permanent		Status: permanent
	Specification document(s): Section 3.3 of this document		Specification document(s): Section 3.3 of this document
	11. References		11. References
	11.1. Normative References		11.1. Normative References
	[CACHING] 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>.
	[MESSAGING]
	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,
	<https://www.rfc-editor.org/info/rfc7230>.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax		[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF", STD 68, RFC 5234,		Specifications: ABNF", STD 68, RFC 5234,
	DOI 10.17487/RFC5234, January 2008,		DOI 10.17487/RFC5234, January 2008,
	<https://www.rfc-editor.org/info/rfc5234>.		<https://www.rfc-editor.org/info/rfc5234>.
	skipping to change at line 1020 ¶		skipping to change at page 23, line 6 ¶
	[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",		[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
	RFC 7405, DOI 10.17487/RFC7405, December 2014,		RFC 7405, DOI 10.17487/RFC7405, December 2014,
	<https://www.rfc-editor.org/info/rfc7405>.		<https://www.rfc-editor.org/info/rfc7405>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[SEMANTICS]		[SEMANTICS]
	Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer		Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
	Protocol (HTTP/1.1): Semantics and Content", RFC 7231,		Semantics", draft-ietf-httpbis-semantics-14 (work in
	DOI 10.17487/RFC7231, June 2014,		progress), January 2021.
	<https://www.rfc-editor.org/info/rfc7231>.
	[UNIX] The Open Group, ., "The Single UNIX Specification, Version
	2 - 6 Vol Set for UNIX 98", February 1997.
	11.2. Informative References		11.2. Informative References
	[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:		[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
	Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,		Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
	<https://www.rfc-editor.org/info/rfc3339>.		<https://www.rfc-editor.org/info/rfc3339>.
	[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status		[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
	Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,		Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
	<https://www.rfc-editor.org/info/rfc6585>.		<https://www.rfc-editor.org/info/rfc6585>.
	[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,		[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
	Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",		Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
	RFC 7234, DOI 10.17487/RFC7234, June 2014,		RFC 7234, DOI 10.17487/RFC7234, June 2014,
	<https://www.rfc-editor.org/info/rfc7234>.		<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.
	11.3. URIs		11.3. URIs
	[1] https://lists.w3.org/Archives/Public/ietf-httpapi-wg/		[1] https://lists.w3.org/Archives/Public/ietf-httpapi-wg/
	[2] https://github.com/ietf-wg-httpapi/ratelimit-headers		[2] https://github.com/ietf-wg-httpapi/ratelimit-headers
	[3] https://github.com/httpwg/http-core/		[3] https://github.com/httpwg/http-core/
	pull/317#issuecomment-585868767		pull/317#issuecomment-585868767
	[4] https://github.com/ioggstream/draft-polli-ratelimit-headers/		[4] https://github.com/ioggstream/draft-polli-ratelimit-headers/
	skipping to change at line 1064 ¶		skipping to change at page 24, line 8 ¶
	[5] https://community.ntppool.org/t/another-ntp-client-failure-		[5] https://community.ntppool.org/t/another-ntp-client-failure-
	story/1014/		story/1014/
	[6] https://lists.w3.org/Archives/Public/ietf-http-		[6] https://lists.w3.org/Archives/Public/ietf-http-
	wg/2019JulSep/0202.html		wg/2019JulSep/0202.html
	[7] https://github.com/ioggstream/draft-polli-ratelimit-headers/		[7] https://github.com/ioggstream/draft-polli-ratelimit-headers/
	issues/34#issuecomment-519366481		issues/34#issuecomment-519366481
	Appendix A. Change Log		Appendix A. Acknowledgements
	RFC EDITOR PLEASE DELETE THIS SECTION.
	Appendix B. Acknowledgements
	Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro		Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro
	Ranellucci, Amos Jeffries, Martin Thomson, Erik Wilde and Mark		Ranellucci, Amos Jeffries, Martin Thomson, Erik Wilde and Mark
	Nottingham for being the initial contributors of these		Nottingham for being the initial contributors of these
	specifications. Kudos to the first community implementors: Aapo		specifications. Kudos to the first community implementors: Aapo
	Talvensaari, Nathan Friedly and Sanyam Dogra.		Talvensaari, Nathan Friedly and Sanyam Dogra.
	Appendix C. RateLimit headers currently used on the web		Appendix B. FAQ
	RFC EDITOR PLEASE DELETE THIS SECTION.
	Commonly used header field names are:
	o "X-RateLimit-Limit", "X-RateLimit-Remaining", "X-RateLimit-Reset";
	o "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:
	o "x-ratelimit-limit-minute", "x-ratelimit-limit-hour", "x-
	ratelimit-limit-day"
	o "x-ratelimit-remaining-minute", "x-ratelimit-remaining-hour", "x-
	ratelimit-remaining-day"
	Here are some interoperability issues:
	o "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" [SEMANTICS] or [RFC3339]
	o different headers, with the same semantic, are used by different
	implementers:
	* X-RateLimit-Limit and X-Rate-Limit-Limit
	* X-RateLimit-Remaining and X-Rate-Limit-Remaining
	* X-RateLimit-Reset and X-Rate-Limit-Reset
	The semantic of RateLimit-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?		1. Why defining standard fields for throttling?
	To simplify enforcement of throttling policies.		To simplify enforcement of throttling policies.
	2. Can I use RateLimit-* in throttled responses (eg with status code		2. Can I use RateLimit-* in throttled responses (eg with status code
	429)?		429)?
	Yes, you can.		Yes, you can.
	3. Are those specs tied to RFC 6585?		3. Are those specs tied to RFC 6585?
	No. [RFC6585] defines the "429" status code and we use it just		No. [RFC6585] defines the "429" status code and we use it just
	as an example of a throttled request, that could instead use even		as an example of a throttled request, that could instead use even
	403 or whatever status code.		"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?		4. Why don't pass the throttling scope as a parameter?
	After a discussion on a similar thread [3] we will probably add a		After a discussion on a similar thread [3] we will probably add a
	new "RateLimit-Scope" header to this spec.		new "RateLimit-Scope" field to this spec.
	I'm open to suggestions: comment on this issue [4]		I'm open to suggestions: comment on this issue [4]
	5. Why using delta-seconds instead of a UNIX Timestamp? Why not		5. Why using delay-seconds instead of a UNIX Timestamp? Why not
	using subsecond precision?		using subsecond precision?
	Using delta-seconds aligns with "Retry-After", which is returned		Using delay-seconds aligns with "Retry-After", which is returned
	in similar contexts, eg on 429 responses.		in similar contexts, eg on 429 responses.
	delta-seconds as defined in [CACHING] section 1.2.1 clarifies
	some parsing rules too.
	Timestamps require a clock synchronization protocol (see		Timestamps require a clock synchronization protocol (see
	[SEMANTICS] section 4.1.1.1). This may be problematic (eg. clock		Section 5.6.7 of [SEMANTICS]). This may be problematic (eg.
	adjustment, clock skew, failure of hardcoded clock		clock adjustment, clock skew, failure of hardcoded clock
	synchronization servers, IoT devices, ..). Moreover timestamps		synchronization servers, IoT devices, ..). Moreover timestamps
	may not be monotonically increasing due to clock adjustment. See		may not be monotonically increasing due to clock adjustment. See
	Another NTP client failure story [5]		Another NTP client failure story [5]
	We did not use subsecond precision because:		We did not use subsecond precision because:
	* that is more subject to system clock correction like the one		* that is more subject to system clock correction like the one
	implemented via the adjtimex() Linux system call;		implemented via the adjtimex() Linux system call;
	* response-time latency may not make it worth. A brief		* response-time latency may not make it worth. A brief
	skipping to change at line 1212 ¶		skipping to change at page 25, line 45 ¶
	A semantic way to limit concurrency is to return 503 + Retry-		A semantic way to limit concurrency is to return 503 + Retry-
	After in case of resource saturation (eg. thrashing, connection		After in case of resource saturation (eg. thrashing, connection
	queues too long, Service Level Objectives not meet, ..).		queues too long, Service Level Objectives not meet, ..).
	Saturation conditions can be either dynamic or static: all this		Saturation conditions can be either dynamic or static: all this
	is out of the scope for the current document.		is out of the scope for the current document.
	8. Do a positive value of "RateLimit-Remaining" imply any service		8. Do a positive value of "RateLimit-Remaining" imply any service
	guarantee for my future requests to be served?		guarantee for my future requests to be served?
	No. The returned values were used to decide whether to serve or		No. FAQ integrated in Section 3.2.
	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?		9. Is the quota-policy definition Section 2.3 too complex?
	You can always return the simplest form of the 3 headers		You can always return the simplest form of the 3 fields
	RateLimit-Limit: 100		RateLimit-Limit: 100
	RateLimit-Remaining: 50		RateLimit-Remaining: 50
	RateLimit-Reset: 60		RateLimit-Reset: 60
	The key runtime value is the first element of the list: "expiring-		The key runtime value is the first element of the list: "expiring-
	limit", the others "quota-policy" are informative. So for the		limit", the others "quota-policy" are informative. So for the
	following header:		following field:
	RateLimit-Limit: 100, 100;w=60;burst=1000;comment="sliding window", 5000;w=3600;burst=0;comment="fixed window"		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"		the key value is the one referencing the lowest limit: "100"
	1. Can we use shorter names? Why don't put everything in one		1. Can we use shorter names? Why don't put everything in one field?
	header?
	The most common syntax we found on the web is "X-RateLimit-*" and		The most common syntax we found on the web is "X-RateLimit-*" and
	when starting this I-D we opted for it [7]		when starting this I-D we opted for it [7]
	The basic form of those headers is easily parseable, even by		The basic form of those fields is easily parseable, even by
	implementors procesing responses using technologies like dynamic		implementors procesing responses using technologies like dynamic
	interpreter with limited syntax.		interpreter with limited syntax.
	Using a single header complicates parsing and takes a significantly		Using a single field complicates parsing and takes a significantly
	different approach from the existing ones: this can limit adoption.		different approach from the existing ones: this can limit adoption.
	1. Why don't mention connections?		1. Why don't mention connections?
	Beware of the term "connection": &#65532; &#65532; - it is just		Beware of the term "connection": &#65532; &#65532; - it is just
	_one_ possible saturation cause. Once you go that path &#65532;		_one_ possible saturation cause. Once you go that path &#65532;
	you will expose other infrastructural details (bandwidth, CPU, ..		you will expose other infrastructural details (bandwidth, CPU, ..
	see Section 9.2) &#65532; and complicate client compliance;		see Section 9.2) &#65532; and complicate client compliance;
	&#65532; - it is an infrastructural detail defined in terms of		&#65532; - it is an infrastructural detail defined in terms of
	server and network &#65532; rather than the consumed service.		server and network &#65532; rather than the consumed service.
	This specification protects the services first, and then the		This specification protects the services first, and then the
	infrastructures through client cooperation (see Section 9.1).		infrastructures through client cooperation (see Section 9.1).
	&#65532; &#65532; RateLimit headers enable sending _on the same		&#65532; &#65532; RateLimit fields enable sending _on the same
	connection_ different limit values &#65532; on each response,		connection_ different limit values &#65532; on each response,
	depending on the policy scope (eg. per-user, per-custom-key, ..)		depending on the policy scope (eg. per-user, per-custom-key, ..)
	&#65532;		&#65532;
	2. Can intermediaries alter RateLimit fields?		2. Can intermediaries alter RateLimit fields?
	Generally, they should not because it might result in unserviced		Generally, they should not because it might result in unserviced
	requests. There are reasonable use cases for intermediaries		requests. There are reasonable use cases for intermediaries
	mangling RateLimit fields though, e.g. when they enforce stricter		mangling RateLimit fields though, e.g. when they enforce stricter
	quota-policies, or when they are an active component of the		quota-policies, or when they are an active component of the
	service. In those case we will consider them as part of the		service. In those case we will consider them as part of the
	originating infrastructure.		originating infrastructure.
			RateLimit fields currently used on the web
			_RFC Editor: Please remove this section before publication._
			Commonly used header field names are:
			o "X-RateLimit-Limit", "X-RateLimit-Remaining", "X-RateLimit-Reset";
			o "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:
			o "x-ratelimit-limit-minute", "x-ratelimit-limit-hour", "x-
			ratelimit-limit-day"
			o "x-ratelimit-remaining-minute", "x-ratelimit-remaining-hour", "x-
			ratelimit-remaining-day"
			Here are some interoperability issues:
			o "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]
			o different headers, with the same semantic, are used by different
			implementers:
			* X-RateLimit-Limit and X-Rate-Limit-Limit
			* X-RateLimit-Remaining and X-Rate-Limit-Remaining
			* X-RateLimit-Reset and X-Rate-Limit-Reset
			The semantic of RateLimit-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._
			D.1. Since draft-ietf-httpapi-ratelimit-headers-00
			o Use I-D.httpbis-semantics, which includes referencing "delay-
			seconds" instead of "delta-seconds". #5
	Authors' Addresses		Authors' Addresses
	Roberto Polli		Roberto Polli
	Team Digitale, Italian Government		Team Digitale, Italian Government
			Italy
	Email: robipolli@gmail.com		Email: robipolli@gmail.com
	Alejandro Martinez Ruiz		Alejandro Martinez Ruiz
	Red Hat		Red Hat
	Email: amr@redhat.com		Email: amr@redhat.com
End of changes. 102 change blocks.
	275 lines changed or deleted		291 lines changed or added
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