draft-ietf-httpapi-ratelimit-headers-01.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: November 12, 2021 Red Hat		Expires: April 19, 2022 Red Hat
	May 11, 2021		October 16, 2021
	RateLimit Header Fields for HTTP		RateLimit Header Fields for HTTP
	draft-ietf-httpapi-ratelimit-headers-01		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 service limits 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://mailarchive.ietf.org/arch/browse/httpapi/ [1].
	The source code and issues list for this draft can be found at		The source code and issues list for this draft can be found at
	https://github.com/ietf-wg-httpapi/ratelimit-headers [2].		https://github.com/ietf-wg-httpapi/ratelimit-headers [2].
			References to "ThisRFC" in the IANA Considerations section would be
			replaced with the RFC number when assigned.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	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 November 12, 2021.		This Internet-Draft will expire on April 19, 2022.
	Copyright Notice		Copyright Notice
	Copyright (c) 2021 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. Service limit		2.2. Service limit . . . . . . . . . . . . . . . . . . . . . . 7
	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 . . . . . . . . . . . . . . . . . . . . . 10
	4. Providing RateLimit fields		4. Providing RateLimit fields . . . . . . . . . . . . . . . . . 10
	4.1. Performance considerations		4.1. Performance considerations . . . . . . . . . . . . . . . 12
	5. Intermediaries		5. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . 12
	6. Caching		6. Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
	7. Receiving RateLimit fields		7. Receiving RateLimit fields . . . . . . . . . . . . . . . . . 13
	8. Examples		8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
	8.1. Unparameterized responses		8.1. Unparameterized responses . . . . . . . . . . . . . . . . 14
	8.1.1. Throttling information in responses		8.1.1. Throttling information in responses . . . . . . . . . 14
	8.1.2. Use in conjunction with custom fields		8.1.2. Use in conjunction with custom fields . . . . . . . . 15
	8.1.3. Use for limiting concurrency		8.1.3. Use for limiting concurrency . . . . . . . . . . . . 16
	8.1.4. Use in throttled responses		8.1.4. Use in throttled responses . . . . . . . . . . . . . 17
	8.2. Parameterized responses		8.2. Parameterized responses . . . . . . . . . . . . . . . . . 17
	8.2.1. Throttling window specified via parameter		8.2.1. Throttling window specified via parameter . . . . . . 17
	8.2.2. Dynamic limits with parameterized windows		8.2.2. Dynamic limits with parameterized windows . . . . . . 18
	8.2.3. Dynamic limits for pushing back and slowing down		8.2.3. Dynamic limits for pushing back and slowing down . . 18
	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 . . . . . . . . . . . . . . . . . . . . . . . . . . 19
	8.3.1. Missing Remaining information		8.3.1. Missing Remaining information . . . . . . . . . . . . 20
	8.3.2. Use with multiple windows		8.3.2. Use with multiple windows . . . . . . . . . . . . . . 21
	9. Security Considerations		9. Security Considerations . . . . . . . . . . . . . . . . . . . 22
	9.1. Throttling does not prevent clients from issuing requests		9.1. Throttling does not prevent clients from issuing requests 22
	9.2. Information disclosure		9.2. Information disclosure . . . . . . . . . . . . . . . . . 22
	9.3. Remaining quota-units are not granted requests		9.3. Remaining quota-units are not granted requests . . . . . 22
	9.4. Reliability of RateLimit-Reset		9.4. Reliability of RateLimit-Reset . . . . . . . . . . . . . 22
	9.5. Resource exhaustion		9.5. Resource exhaustion . . . . . . . . . . . . . . . . . . . 23
	9.6. Denial of Service		9.6. Denial of Service . . . . . . . . . . . . . . . . . . . . 23
	10. IANA Considerations		10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
	10.1. RateLimit-Limit Field Registration		10.1. RateLimit Parameters Registration . . . . . . . . . . . 24
	10.2. RateLimit-Remaining Field Registration		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
	10.3. RateLimit-Reset Field Registration		11.1. Normative References . . . . . . . . . . . . . . . . . . 25
	11. References		11.2. Informative References . . . . . . . . . . . . . . . . . 26
	11.1. Normative References		11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 26
	11.2. Informative References		Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 27
	11.3. URIs		Appendix B. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 27
	Appendix A. Acknowledgements		RateLimit fields currently used on the web . . . . . . . . . . . 30
	Appendix B. FAQ		Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
	RateLimit fields currently used on the web		D.1. Since draft-ietf-httpapi-ratelimit-headers-00 . . . . . . 31
	Changes		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
	D.1. Since draft-ietf-httpapi-ratelimit-headers-00
	Authors' Addresses
	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 by 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" (see		defined in [SEMANTICS] to be returned in "429 Too Many Requests" (see
	[STATUS429]) or "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 - e.g. monetization.
	A basic quota mechanism limits the number of acceptable requests in a		A basic quota mechanism limits the number of acceptable requests in a
	given time window, eg. 10 requests per second.		given time window, e.g. 10 requests per second.
	When quota is exceeded, servers usually do not serve the request		When quota is exceeded, servers usually do not serve the request
	replying instead with a "4xx" HTTP status code (eg. 429 or 403) or		replying instead with a "4xx" HTTP status code (e.g. 429 or 403) or
	adopt more aggressive policies like dropping connections.		adopt more aggressive policies like dropping connections.
	Quotas may be enforced on different basis (eg. per user, per IP, per		Quotas may be enforced on different basis (e.g. per user, per IP, per
	geographic area, ..) and at different levels. For example, an user		geographic area, ..) and at different levels. For example, an user
	may be allowed to issue:		may be allowed to issue:
	o 10 requests per second;		o 10 requests per second;
	o limited to 60 request per minute;		o limited to 60 requests per minute;
	o limited to 1000 request per hour.		o limited to 1000 requests per hour.
	Moreover system metrics, statistics and heuristics can be used to		Moreover system metrics, statistics and heuristics can be used to
	implement more complex policies, where the number of acceptable		implement more complex policies, where the number of acceptable
	request and the time window are computed dynamically.		requests and the time window are computed dynamically.
	1.2. Current landscape of rate-limiting headers		1.2. Current landscape of rate-limiting headers
	To help clients throttling their requests, servers may expose the		To help clients throttling their requests, servers may expose the
	counters used to evaluate quota policies via HTTP header fields.		counters used to evaluate quota policies via HTTP header fields.
	Those response headers may be added by HTTP intermediaries such as		Those response headers may be added by HTTP intermediaries such as
	API gateways and reverse proxies.		API gateways and reverse proxies.
	On the web we can find many different rate-limit headers, usually		On the web we can find many different rate-limit headers, usually
	skipping to change at line 220 ¶		skipping to change at page 5, line 34 ¶
	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 semantics of rate-limit headers to		Interoperability: Standardization of the names and semantics of
	ease their enforcement and adoption;		rate-limit headers to ease their enforcement and adoption;
	2. Improve resiliency of HTTP infrastructure by providing clients		Resiliency: Improve resiliency of HTTP infrastructure by providing
	with information useful to throttle their requests and prevent		clients with information useful to throttle their requests and
	4xx or 5xx responses;		prevent 4xx or 5xx responses;
	3. Simplify API documentation by eliminating the need to include		Documentation: Simplify API documentation by eliminating the need to
	detailed quota limits and related header fields in API		include detailed quota limits and related header fields in API
	documentation.		documentation.
	The goals do not include:		The goals do not include:
	Authorization: The rate-limit fields described here are not meant to		Authorization: The rate-limit fields described here are not meant 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 (see Section 7 of [RFC6454]).		the whole Origin (see Section 7 of [RFC6454]).
	skipping to change at line 270 ¶		skipping to change at page 6, line 35 ¶
	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		by [RFC7405] along with the "#rule" extension defined in
	Section 5.6.1 of [SEMANTICS].		Section 5.6.1 of [SEMANTICS].
	The term Origin is to be interpreted as described in Section 7 of		The term Origin is to be interpreted as described in Section 7 of
	[RFC6454].		[RFC6454].
	The "delay-seconds" rule is defined in Section 10.2.4 of [SEMANTICS].		This specification uses Structured Fields [SF] to specify syntax.
			The terms sf-list, sf-item, sf-string, sf-token, sf-integer, bare-
			item and key refer to the structured types defined therein.
	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 = delay-seconds		time-window = delay-seconds
			delay-seconds = sf-integer
			Where "delay-seconds" is a non-negative sf-integer compatible with
			the "delay-seconds" rule defined in Section 10.2.3 of [SEMANTICS].
	Subsecond precision is not supported.		Subsecond precision is not supported.
	2.2. Service limit		2.2. Service limit
	The service-limit 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 "service-limit" is expressed in "quota-units" and has the		The "service-limit" is expressed in "quota-units" and has the
	following syntax:		following syntax:
	service-limit = quota-units		service-limit = quota-units
	quota-units = 1*DIGIT		quota-units = sf-integer
			where quota-units is a non-negative sf-integer.
	The "service-limit" SHOULD match the maximum number of acceptable		The "service-limit" SHOULD match the maximum number of acceptable
	requests.		requests.
	The "service-limit" 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 "service-limit" 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=WuMing"
	so that we have the following counters		so that we have the following counters
	GET /books/123 ; service-limit=4, remaining: 3, status=200		GET /books/123 ; service-limit=4, remaining: 3, status=200
	GET /books?author=Camilleri ; service-limit=4, remaining: 1, status=200		GET /books?author=WuMing ; service-limit=4, remaining: 1, status=200
	GET /books?author=Eco ; service-limit=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 = service-limit; "w" "=" time-window		quota-policy = sf-item
	*( OWS ";" OWS quota-comment)
	quota-comment = token "=" (token / quoted-string)
	quota-policy parameters like "w" and quota-comment tokens MUST NOT		where the associated bare-item is a service-limit and parameters are
	occur multiple times within the same quota-policy.		supported.
			The following parameters are defined:
			w: The "w" parameter specifies a time window. Its syntax is a "time-
			window" defined in Section 2.1.
			Other parameters are allowed and can be regarded as comments. They
			ought to be registered within the "Hypertext Transfer Protocol (HTTP)
			RateLimit Parameters Registry", as described in Section 10.1.
	An example policy of 100 quota-units per minute.		An example policy of 100 quota-units per minute.
	100;w=60		100;w=60
	The definition of a quota-policy does not imply any specific		The definition of a quota-policy does not imply any specific
	distribution of quota-units over time. Such service specific details		distribution of quota-units over time. Such service specific details
	can be conveyed in the "quota-comments".		can be conveyed as parameters.
	Two examples of providing further details via custom parameters in		Two examples of providing further details via custom parameters
	"quota-comments".
	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 "service-limit"		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 field value is		The field is a List Structured Field of positive length. The first
			member is named "expiring-limit" and its syntax is "service-limit",
			while the syntax of the other optional members is "quota-policy"
	RateLimit-Limit = expiring-limit [, 1#quota-policy ]		RateLimit-Limit = sf-list
	expiring-limit = service-limit
	The "expiring-limit" value MUST be set to the "service-limit" 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
	skipping to change at line 383 ¶		skipping to change at page 9, line 22 ¶
	example		example
	RateLimit-Limit: 100, 100;w=10		RateLimit-Limit: 100, 100;w=10
	If the "expiring-limit" is not associated to a "time-window", the		If the "expiring-limit" is not associated to a "time-window", the
	"time-window" MUST either be:		"time-window" MUST either be:
	o inferred by the value of "RateLimit-Reset" at the moment of the		o inferred by the value of "RateLimit-Reset" at the moment of the
	reset, or		reset, or
	o communicated out-of-band (eg. in the documentation).		o communicated out-of-band (e.g. 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 field MUST NOT occur multiple times and can be sent in a trailer		This field MUST NOT occur multiple times and can be sent in a 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 field value is		The field is an Integer Structured Field and its value is
	RateLimit-Remaining = quota-units		RateLimit-Remaining = quota-units
	This field MUST NOT occur multiple times and can be sent in a trailer		This field MUST NOT occur multiple times and can be sent in a 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 that further requests will be served.		is a guarantee that further requests will be served.
	A low "RateLimit-Remaining" value is like a yellow traffic-light for		A low "RateLimit-Remaining" value is like a yellow traffic-light for
	skipping to change at line 424 ¶		skipping to change at page 10, line 15 ¶
	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 field value is		The field is an Integer Structured Field and its value is
	RateLimit-Reset = delay-seconds		RateLimit-Reset = delay-seconds
	The delay-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
	Section 5.6.7 of [SEMANTICS]);		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
	skipping to change at line 447 ¶		skipping to change at page 10, line 38 ¶
	This field MUST NOT occur multiple times and can be sent in a trailer		This field MUST NOT occur multiple times and can be sent in a 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 "service-limit" 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 e.g. in case of resource saturation or to
	implement sliding window policies.		implement sliding window policies.
	4. Providing RateLimit fields		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.
	skipping to change at line 460 ¶		skipping to change at page 11, line 4 ¶
	4. Providing RateLimit fields		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
	RateLimit-Limit: 10		RateLimit-Limit: 10
	RateLimit-Remaining: 1		RateLimit-Remaining: 1
	RateLimit-Reset: 7		RateLimit-Reset: 7
	does not guarantee that the next request will be successful. Server		does not guarantee that the next request will be successful. Server
	metrics may be subject to other conditions like the one shown in the		metrics may be subject to other conditions like the one shown in the
	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.
	This document does not mandate any correlation between the		This document does not mandate any correlation between the
	"RateLimit" values and the returned status code.		"RateLimit" values and the returned status code.
	Servers should be careful in returning "RateLimit" fields in		Servers should be careful in returning "RateLimit" fields in
	redirection responses (eg. 3xx status codes) because a low		redirection responses (e.g. 3xx status codes) because a low
	"RateLimit-Remaining" value could limit the client from issuing		"RateLimit-Remaining" value could limit the client from issuing
	requests. For example, given the rate limiting fields below, a		requests. For example, given the rate limiting fields below, a
	client could decide to wait 10 seconds before following the		client could decide to wait 10 seconds before following the
	"Location" header, because "RateLimit-Remaining" is 0.		"Location" header, because "RateLimit-Remaining" is 0.
	HTTP/1.1 301 Moved Permanently		HTTP/1.1 301 Moved Permanently
	Location: /foo/123		Location: /foo/123
	RateLimit-Remaining: 0		RateLimit-Remaining: 0
	RateLimit-Limit: 10		RateLimit-Limit: 10
	RateLimit-Reset: 10		RateLimit-Reset: 10
	skipping to change at line 503 ¶		skipping to change at page 11, line 46 ¶
	MUST reply with the "RateLimit" fields related to the window with the		MUST reply with the "RateLimit" fields related to the window with the
	lower "RateLimit-Remaining" values.		lower "RateLimit-Remaining" values.
	A service returning "RateLimit" fields MUST NOT convey values		A service returning "RateLimit" fields MUST NOT convey values
	exposing an unwanted volume of requests and SHOULD implement		exposing an unwanted volume of requests and SHOULD implement
	mechanisms to cap the ratio between "RateLimit-Remaining" and		mechanisms to cap the ratio between "RateLimit-Remaining" and
	"RateLimit-Reset" (see Section 9.5); this is especially important		"RateLimit-Reset" (see Section 9.5); this is especially important
	when quota-policies use a large "time-window".		when quota-policies use a large "time-window".
	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, e.g. to respond to Denial
	Service attacks or in case of resource saturation.		of 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.
	4.1. Performance considerations		4.1. Performance considerations
	Servers are not required to return "RateLimit" fields in every		Servers are not required to return "RateLimit" fields in every
	response, and clients need to take this into account. For example,		response, and clients need to take this into account. For example,
	an implementer concerned with performance might provide "RateLimit"		an implementer concerned with performance might provide "RateLimit"
	fields only when a given quota is going to expire.		fields only when a given quota is going to expire.
	Implementers concerned with response fields' size, might take into		Implementers concerned with response fields' size, might take into
	account their ratio with respect to the payload data, or use header-		account their ratio with respect to the payload data, or use header-
	compression http features such as [HPACK].		compression http features such as [HPACK].
	5. Intermediaries		5. Intermediaries
	This section documents the considerations advised in Section 16.3.3		This section documents the considerations advised in Section 16.3.2
	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 584 ¶		skipping to change at page 13, line 37 ¶
	Malformed "RateLimit" fields 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 information (eg. the "Date" response header field, or		and use other information (e.g. 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, "Retry-After" MUST take precedence and "RateLimit-Reset" MAY		fields, "Retry-After" MUST take precedence and "RateLimit-Reset" MAY
	be ignored.		be ignored.
	This specification does not mandate a specific throttling behavior		This specification does not mandate a specific throttling behavior
	and implementers can adopt their preferred policies, including:		and implementers can adopt their preferred policies, including:
	o slowing down or preemptively backoff their request rate when		o slowing down or preemptively back-off their request rate when
	approaching quota limits;		approaching quota limits;
	o consuming all the quota according to the exposed limits and then		o consuming all the quota according to the exposed limits and then
	wait.		wait.
	8. Examples		8. Examples
	8.1. Unparameterized responses		8.1. Unparameterized responses
	8.1.1. Throttling information in responses		8.1.1. Throttling information in responses
	skipping to change at line 857 ¶		skipping to change at page 19, line 33 ¶
	"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 "Retry-		starts, we can convey the same information to the client via "Retry-
	After", with the advantage that the server can now specify the		After", with the advantage that the server can now specify the
	policy's nominal limit and window that will apply after the reset,		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		e.g. assuming the resource exhaustion is likely to be gone by then,
	the advertised policy does not need to be adjusted, yet we managed to		so the advertised policy does not need to be adjusted, yet we managed
	stop requests for a while and slow down the rest of the current		to stop requests for a while and slow down the rest of the current
	window.		window.
	Request:		Request:
	GET /items/123 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: api.example		Host: api.example
	Response:		Response:
	HTTP/1.1 429 Too Many Requests		HTTP/1.1 429 Too Many Requests
	skipping to change at line 890 ¶		skipping to change at page 20, line 26 ¶
	Note that in this last response the client is expected to honor		Note that in this last response the client is expected to honor
	"Retry-After" and perform no requests for the specified amount of		"Retry-After" and perform no requests for the specified amount of
	time, whereas the previous example would not force the client to stop		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		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		to query again the server even if it is likely to have the request
	rejected.		rejected.
	8.3.1. Missing Remaining information		8.3.1. Missing Remaining information
	The server does not expose "RateLimit-Remaining" values, but resets		The server does not expose "RateLimit-Remaining" values (for example,
	the limit counter every second.		because the underlying counters are not available). Instead, it
			resets the limit counter every second.
	It communicates to the client the limit of 10 quota-units per second		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 HTTP/1.1		GET /items/123 HTTP/1.1
	Host: api.example		Host: api.example
	Response:		Response:
	skipping to change at line 1064 ¶		skipping to change at page 24, line 14 ¶
	o passed by a misconfigured server;		o passed by a misconfigured server;
	or an high "RateLimit-Reset" value could inhibit clients to contact		or an high "RateLimit-Reset" value could inhibit clients to contact
	the server.		the server.
	Clients MUST validate the received values to mitigate those risks.		Clients MUST validate the received values to mitigate those risks.
	10. IANA Considerations		10. IANA Considerations
	10.1. RateLimit-Limit Field Registration		IANA is requested to update one registry and create one new registry.
	This section registers the "RateLimit-Limit" field in the "Hypertext
	Transfer Protocol (HTTP) Field Name Registry" registry ([SEMANTICS]).
	Field name: "RateLimit-Limit"
	Status: permanent		Please add the following entries to the "Hypertext Transfer Protocol
			(HTTP) Field Name Registry" registry ([SEMANTICS]):
	Specification document(s): Section 3.1 of this document		+---------------------+-----------+------------------------+
			| Field Name | Status | Specification |
			+---------------------+-----------+------------------------+
			| RateLimit-Limit | permanent | Section 3.1 of ThisRFC |
			| RateLimit-Remaining | permanent | Section 3.2 of ThisRFC |
			| RateLimit-Reset | permanent | Section 3.3 of ThisRFC |
			+---------------------+-----------+------------------------+
	10.2. RateLimit-Remaining Field Registration		10.1. RateLimit Parameters Registration
	This section registers the "RateLimit-Remaining" field in the		IANA is requested to create a new registry to be called "Hypertext
	"Hypertext Transfer Protocol (HTTP) Field Name Registry" registry		Transfer Protocol (HTTP) RateLimit Parameters Registry", to be
	([SEMANTICS]).		located at https://www.iana.org/assignments/http-ratelimit-parameters
			[3]. Registration is done on the advice of a Designated Expert,
			appointed by the IESG or their delegate. All entries are
			Specification Required ([IANA], Section 4.6).
	Field name: "RateLimit-Remaining"		Registration requests consist of the following information:
	Status: permanent		o Parameter name: The parameter name, conforming to [SF].
	Specification document(s): Section 3.2 of this document		o Field name: The RateLimit field for which the parameter is
			registered. If a parameter is intended to be used with multiple
			fields, it has to be registered for each one.
	10.3. RateLimit-Reset Field Registration		o Description: A brief description of the parameter.
	This section registers the "RateLimit-Reset" field in the "Hypertext		o Specification document: A reference to the document that specifies
	Transfer Protocol (HTTP) Field Name Registry" registry ([SEMANTICS]).		the parameter, preferably including a URI that can be used to
			retrieve a copy of the document.
	Field name: "RateLimit-Reset"		o Comments (optional): Any additional information that can be
			useful.
	Status: permanent		The initial contents of this registry should be:
	Specification document(s): Section 3.3 of this document		+---------------+----------+------------+--------------+------------+
			| Field Name | Paramete | Descriptio | Specificatio | Comments |
			| | r name | n | n | (optional) |
			+---------------+----------+------------+--------------+------------+
			| RateLimit- | w | Time | Section 2.3 | |
			| Limit | | window | of ThisRFC | |
			+---------------+----------+------------+--------------+------------+
	11. References		11. References
	11.1. Normative References		11.1. Normative References
			[IANA] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
			Writing an IANA Considerations Section in RFCs", BCP 26,
			RFC 8126, DOI 10.17487/RFC8126, June 2017,
			<https://www.rfc-editor.org/info/rfc8126>.
	[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 1126 ¶		skipping to change at page 25, line 48 ¶
	[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. T., Nottingham, M., and J. Reschke, "HTTP		Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
	Semantics", draft-ietf-httpbis-semantics-15 (work in		Semantics", draft-ietf-httpbis-semantics-19 (work in
	progress), March 2021.		progress), September 2021.
			[SF] Nottingham, M. and P-H. Kamp, "Structured Field Values for
			HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
			<https://www.rfc-editor.org/info/rfc8941>.
	11.2. Informative References		11.2. Informative References
	[HPACK] Peon, R. and H. Ruellan, "HPACK: Header Compression for		[HPACK] Peon, R. and H. Ruellan, "HPACK: Header Compression for
	HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,		HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
	<https://www.rfc-editor.org/info/rfc7541>.		<https://www.rfc-editor.org/info/rfc7541>.
	[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>.
	skipping to change at line 1159 ¶		skipping to change at page 26, line 39 ¶
	Stewart, R., Tuexen, M., and P. Lei, "Stream Control		Stewart, R., Tuexen, M., and P. Lei, "Stream Control
	Transmission Protocol (SCTP) Stream Reconfiguration",		Transmission Protocol (SCTP) Stream Reconfiguration",
	RFC 6525, DOI 10.17487/RFC6525, February 2012,		RFC 6525, DOI 10.17487/RFC6525, February 2012,
	<https://www.rfc-editor.org/info/rfc6525>.		<https://www.rfc-editor.org/info/rfc6525>.
	[UNIX] The Open Group, ., "The Single UNIX Specification, Version		[UNIX] The Open Group, ., "The Single UNIX Specification, Version
	2 - 6 Vol Set for UNIX 98", February 1997.		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://mailarchive.ietf.org/arch/browse/httpapi/
	[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://www.iana.org/assignments/http-ratelimit-parameters
			[4] https://github.com/httpwg/http-core/
	pull/317#issuecomment-585868767		pull/317#issuecomment-585868767
	[4] https://github.com/ioggstream/draft-polli-ratelimit-headers/		[5] https://github.com/ioggstream/draft-polli-ratelimit-headers/
	issues/70		issues/70
	[5] https://community.ntppool.org/t/another-ntp-client-failure-		[6] https://community.ntppool.org/t/another-ntp-client-failure-
	story/1014/		story/1014/
	[6] https://lists.w3.org/Archives/Public/ietf-http-		[7] 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/		[8] https://github.com/ioggstream/draft-polli-ratelimit-headers/
	issues/34#issuecomment-519366481		issues/34#issuecomment-519366481
	Appendix A. Acknowledgements		Appendix A. 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 implementers: Aapo
	Talvensaari, Nathan Friedly and Sanyam Dogra.		Talvensaari, Nathan Friedly and Sanyam Dogra.
	Appendix B. FAQ		Appendix B. FAQ
	1. Why defining standard fields 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)?
	skipping to change at line 1209 ¶		skipping to change at page 27, line 42 ¶
	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. The goal of this specification is		"403" or whatever status code. The goal of this specification is
	to standardize the name and semantic of three ratelimit fields		to standardize the name and semantic of three ratelimit fields
	widely used on the internet. Stricter relations with status		widely used on the internet. Stricter relations with status
	codes or error response payloads would impose behaviors to all		codes or error response payloads would impose behaviors to all
	the existing implementations making the adoption more complex.		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 [4] we will probably add a
	new "RateLimit-Scope" field 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 [5]
	5. Why using delay-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 delay-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.
	Timestamps require a clock synchronization protocol (see		Timestamps require a clock synchronization protocol (see
	Section 5.6.7 of [SEMANTICS]). This may be problematic (eg.		Section 5.6.7 of [SEMANTICS]). This may be problematic (e.g.
	clock 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 [6]
	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
	discussion on the subject is on the httpwg ml [6]		discussion on the subject is on the httpwg ml [7]
	* almost all rate-limit headers implementations do not use it.		* almost all rate-limit headers implementations do not use it.
	6. Why not support multiple quota remaining?		6. Why not support multiple quota remaining?
	While this might be of some value, my experience suggests that		While this might be of some value, my experience suggests that
	overly-complex quota implementations results in lower		overly-complex quota implementations results in lower
	effectiveness of this policy. This spec allows the client to		effectiveness of this policy. This spec allows the client to
	easily focusing on RateLimit-Remaining and RateLimit-Reset.		easily focusing on RateLimit-Remaining and RateLimit-Reset.
	7. Shouldn't I limit concurrency instead of request rate?		7. Shouldn't I limit concurrency instead of request rate?
	You can use this specification to limit concurrency at the HTTP		You can use this specification to limit concurrency at the HTTP
	level (see {#use-for-limiting-concurrency}) and help clients to		level (see {#use-for-limiting-concurrency}) and help clients to
	shape their requests avoiding being throttled out.		shape their requests avoiding being throttled out.
	A problematic way to limit concurrency is connection dropping,		A problematic way to limit concurrency is connection dropping,
	especially when connections are multiplexed (eg. HTTP/2) because		especially when connections are multiplexed (e.g. HTTP/2)
	this results in unserviced client requests, which is something we		because this results in unserviced client requests, which is
	want to avoid.		something we want to avoid.
	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 (e.g. 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. FAQ integrated in Section 3.2.		No. FAQ integrated in Section 3.2.
	9. Is the quota-policy definition Section 2.3 too complex?		9. Is the quota-policy definition Section 2.3 too complex?
	skipping to change at line 1267 ¶		skipping to change at page 29, line 4 ¶
	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. FAQ integrated in Section 3.2.		No. FAQ integrated in Section 3.2.
	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 fields		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 field:		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 field?		1. Can we use shorter names? Why don't put everything in one field?
	The most common syntax we found on the web is "X-RateLimit-*" and		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 [8]
	The basic form of those fields is easily parseable, even by		The basic form of those fields is easily parseable, even by
	implementors procesing responses using technologies like dynamic		implementers processing responses using technologies like dynamic
	interpreter with limited syntax.		interpreter with limited syntax.
	Using a single field 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 fields 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 (e.g. 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.
	skipping to change at line 1374 ¶		skipping to change at page 31, line 17 ¶
	* X-RateLimit-Limit and X-Rate-Limit-Limit		* X-RateLimit-Limit and X-Rate-Limit-Limit
	* X-RateLimit-Remaining and X-Rate-Limit-Remaining		* X-RateLimit-Remaining and X-Rate-Limit-Remaining
	* X-RateLimit-Reset and X-Rate-Limit-Reset		* X-RateLimit-Reset and X-Rate-Limit-Reset
	The semantic of RateLimit-Remaining depends on the windowing		The semantic of RateLimit-Remaining depends on the windowing
	algorithm. A sliding window policy for example may result in having		algorithm. A sliding window policy for example may result in having
	a "RateLimit-Remaining" value related to the ratio between the		a "RateLimit-Remaining" value related to the ratio between the
	current and the maximum throughput. Eg.		current and the maximum throughput. e.g.
	RateLimit-Limit: 12, 12;w=1		RateLimit-Limit: 12, 12;w=1
	RateLimit-Remaining: 6 ; using 50% of throughput, that is 6 units/s		RateLimit-Remaining: 6 ; using 50% of throughput, that is 6 units/s
	RateLimit-Reset: 1		RateLimit-Reset: 1
	If this is the case, the optimal solution is to achieve		If this is the case, the optimal solution is to achieve
	RateLimit-Limit: 12, 12;w=1		RateLimit-Limit: 12, 12;w=1
	RateLimit-Remaining: 1 ; using 100% of throughput, that is 12 units/s		RateLimit-Remaining: 1 ; using 100% of throughput, that is 12 units/s
	RateLimit-Reset: 1		RateLimit-Reset: 1
End of changes. 74 change blocks.
	151 lines changed or deleted		192 lines changed or added
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