draft-ietf-httpbis-proxy-status-08.txt		draft-ietf-httpbis-proxy-status-latest.txt
	HTTP Working Group M. Nottingham		HTTP Working Group M. Nottingham
	Internet-Draft Fastly		Internet-Draft Fastly
	Intended status: Standards Track P. Sikora		Intended status: Standards Track P. Sikora
	Expires: April 16, 2022 Google		Expires: April 16, 2026 Google
	October 13, 2021		October 13, 2025
	The Proxy-Status HTTP Response Header Field		The Proxy-Status HTTP Response Header Field
	draft-ietf-httpbis-proxy-status-08		draft-ietf-httpbis-proxy-status-latest
	Abstract		Abstract
	This document defines the Proxy-Status HTTP field to convey the		This document defines the Proxy-Status HTTP response field to convey
	details of intermediary response handling, including generated		the details of an intermediary's response handling, including
	errors.		generated errors.
	Note to Readers		About This Document
	_RFC EDITOR: please remove this section before publication_		This note is to be removed before publishing as an RFC.
	Discussion of this draft takes place on the HTTP working group		Status information for this document may be found at
	mailing list (ietf-http-wg@w3.org), which is archived at		<https://datatracker.ietf.org/doc/draft-ietf-httpbis-proxy-status/>.
	https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
	Working Group information can be found at https://httpwg.org/ [2];		Discussion of this document takes place on the HTTP Working Group
	source code and issues list for this draft can be found at		mailing list (<mailto:ietf-http-wg@w3.org>), which is archived at
	https://github.com/httpwg/http-extensions/labels/proxy-status [3].		<https://lists.w3.org/Archives/Public/ietf-http-wg/>. Working Group
			information can be found at <https://httpwg.org/>.
			Source for this draft and an issue tracker can be found at
			<https://github.com/httpwg/http-extensions/labels/proxy-status>.
	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-
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	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
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	This Internet-Draft will expire on April 16, 2022.		This Internet-Draft will expire on April 16, 2026.
	Copyright Notice		Copyright Notice
	Copyright (c) 2021 IETF Trust and the persons identified as the		Copyright (c) 2025 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	1. Introduction		1. Introduction
	HTTP intermediaries (see Section 3.7 of [HTTP]) -- including both		HTTP intermediaries (see Section 3.7 of [HTTP]) -- including both
	forward proxies and gateways (also known as "reverse proxies") --		forward proxies and gateways (also known as "reverse proxies") --
	have become an increasingly significant part of HTTP deployments. In		have become an increasingly significant part of HTTP deployments. In
	particular, reverse proxies and Content Delivery Networks (CDNs) form		particular, reverse proxies and content delivery networks (CDNs) form
	part of the critical infrastructure of many Web sites.		part of the critical infrastructure of many websites.
	Typically, HTTP intermediaries forward requests towards the origin		Typically, HTTP intermediaries forward requests towards the origin
	server (inbound) and then forward their responses back to clients		server (inbound) and then forward their responses back to clients
	(outbound). However, if an error occurs before a response is		(outbound). However, if an error occurs before a response is
	obtained from an inbound server, the response is often generated by		obtained from an inbound server, the response is often generated by
	the intermediary itself.		the intermediary itself.
	HTTP accommodates these types of errors with a few status codes; for		HTTP accommodates these types of errors with a few status codes --
	example, 502 Bad Gateway and 504 Gateway Timeout. However,		for example, 502 (Bad Gateway) and 504 (Gateway Timeout). However,
	experience has shown that more information is necessary to aid		experience has shown that more information is necessary to aid
	debugging and communicate what's happened to the client.		debugging and communicate what's happened to the client.
	Additionally, intermediaries sometimes want to convey additional		Additionally, intermediaries sometimes want to convey additional
	information about their handling of a response, even if they did not		information about their handling of a response, even if they did not
	generate it.		generate it.
	To enable these uses, Section 2 defines a new HTTP response field to		To enable these uses, Section 2 defines a new HTTP response field to
	allow intermediaries to convey details of their handling of a		allow intermediaries to convey details of their handling of a
	response. Section 2.1 enumerates the information that can be added		response. Section 2.1 enumerates the information that can be added
	to the field by intermediaries, which can be extended as per		to the field by intermediaries, which can be extended per
	Section 2.2. Section 2.3 defines a set of error types for use when a		Section 2.2. Section 2.3 defines a set of error types for use when a
	proxy encounters an issue when obtaining a response for the request;		proxy encounters an issue when obtaining a response for the request;
	these can likewise be extended as per Section 2.4.		these can likewise be extended per Section 2.4.
	1.1. Notational Conventions		1.1. 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 BCP		"OPTIONAL" in this document are to be interpreted as described in
	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 specification uses Structured Fields [STRUCTURED-FIELDS] to		This document uses the following terminology from Section 3 of
	specify syntax and parsing, and ABNF [RFC5234] as a shorthand for		[STRUCTURED-FIELDS] to specify syntax and parsing: List, String,
	that syntax. The terms sf-list, sf-item, sf-string, sf-token, sf-		Token, Integer, and Byte Sequence.
	integer and key refer to the structured types defined therein.
	Note that in this specification, "proxy" is used to indicate both		Note that in this specification, "proxy" is used to indicate both
	forward and reverse proxies, otherwise known as gateways. "Next hop"		forward and reverse proxies, otherwise known as gateways. "Next hop"
	indicates the connection in the direction leading to the origin		indicates the connection in the direction leading to the origin
	server for the request.		server for the request.
	2. The Proxy-Status HTTP Field		2. The Proxy-Status HTTP Field
	The Proxy-Status HTTP response field allows an intermediary to convey		The Proxy-Status HTTP response field allows an intermediary to convey
	additional information about its handling of a response and its		additional information about its handling of a response and its
	associated request. The syntax of this header field conforms to		associated request.
	[STRUCTURED-FIELDS].
	It is a List ([STRUCTURED-FIELDS], Section 3.1):
	Proxy-Status = sf-list
	Each member of the list represents an intermediary that has handled		Its value is a List (see Section 3.1 of [STRUCTURED-FIELDS]). Each
	the response. The first member of the list represents the		member of the List represents an intermediary that has handled the
	intermediary closest to the origin server, and the last member of the		response. The first member represents the intermediary closest to
	list represents the intermediary closest to the user agent.		the origin server, and the last member represents the intermediary
			closest to the user agent.
	For example:		For example:
	Proxy-Status: revproxy1.example.net, ExampleCDN		Proxy-Status: revproxy1.example.net, ExampleCDN
	indicates that this response was handled first by		indicates that this response was handled first by
	revproxy1.example.net (a reverse proxy adjacent to the origin server)		revproxy1.example.net (a reverse proxy adjacent to the origin server)
	and then ExampleCDN.		and then ExampleCDN.
	Intermediaries determine when it is appropriate to add the Proxy-		Intermediaries determine when it is appropriate to add the Proxy-
	Status field to a response. Some might decide to append to it to all		Status field to a response. Some might decide to append it to all
	responses, whereas others might only do so when specifically		responses, whereas others might only do so when specifically
	configured to, or when the request contains a header field that		configured to or when the request contains a header field that
	activates a debugging mode.		activates a debugging mode.
	Each member of the list identifies the intermediary that inserted the		Each member of the List identifies the intermediary that inserted the
	value, and MUST have a type of either sf-string or sf-token.		value and MUST have a type of either String or Token . Depending on
	Depending on the deployment, this might be a service name (but not a		the deployment, this might be a service name (but not a software or
	software or hardware product name; e.g., "Example CDN"is appropriate,		hardware product name; e.g., "ExampleCDN" is appropriate, but
	but "ExampleProxy" is not, because it doesn't identify the		"ExampleProxy" is not because it doesn't identify the deployment), a
	deployment), a hostname ("proxy-3.example.com"), an IP address, or a		hostname ("proxy-3.example.com"), an IP address, or a generated
	generated string.		string.
	Parameters on each member (as per Section 3.1.2 of		Parameters of each member (per Section 3.1.2 of [STRUCTURED-FIELDS])
	[STRUCTURED-FIELDS]) convey additional information about that		convey additional information about that intermediary's handling of
	intermediary's handling of the response and its associated request;		the response and its associated request; see Section 2.1. While all
	see Section 2.1. While all of these parameters are OPTIONAL,		of these parameters are OPTIONAL, intermediaries are encouraged to
	intermediaries are encouraged to provide as much information as		provide as much information as possible (but see Section 4 for
	possible (but see Section 4 for security considerations in doing so).		security considerations in doing so).
	When adding a value to the Proxy-Status field, intermediaries SHOULD		When adding a value to the Proxy-Status field, intermediaries SHOULD
	preserve the existing members of the field to allow debugging of the		preserve the existing members of the field to allow debugging of the
	entire chain of intermediaries handling the request, unless		entire chain of intermediaries handling the request unless explicitly
	explicitly configured to remove them (e.g., to prevent internal		configured to remove them (e.g., to prevent internal network details
	network details from leaking; see Section 4).		from leaking; see Section 4).
	Origin servers MUST NOT generate the Proxy-Status field.		Origin servers MUST NOT generate the Proxy-Status field.
	Proxy-Status MAY be sent as a HTTP trailer field. For example, if an		Proxy-Status MAY be sent as an HTTP trailer field. For example, if
	intermediary is streaming a response and the inbound connection		an intermediary is streaming a response and the inbound connection
	suddenly terminates, Proxy-Status can only be appended to the trailer		suddenly terminates, Proxy-Status can only be appended to the trailer
	section of the outbound message, since the header section has already		section of the outbound message since the header section has already
	been sent. However, because it might be silently discarded along the		been sent. However, because it might be silently discarded along the
	path to the user agent (as is the case for all trailer fields; see		path to the user agent (as is the case for all trailer fields; see
	Section 6.5 of [HTTP]), Proxy-Status SHOULD NOT be sent as a trailer		Section 6.5 of [HTTP]), Proxy-Status SHOULD NOT be sent as a trailer
	field unless it is not possible to send it in the header section.		field unless it is not possible to send it in the header section.
	To allow recipients to reconstruct the relative ordering of Proxy-		To allow recipients to reconstruct the relative ordering of Proxy-
	Status members conveyed in trailer fields with those conveyed in		Status members conveyed in trailer fields with those conveyed in
	header fields, an intermediary MUST NOT send Proxy-Status as a		header fields, an intermediary MUST NOT send Proxy-Status as a
	trailer field unless it has also generated a Proxy-Status header		trailer field unless it has also generated a Proxy-Status header
	field with the same member (although potentially different		field with the same member (although potentially different
	skipping to change at page 5, line 4 ¶		skipping to change at page 4, line 46 ¶
	parameters) in that message.		parameters) in that message.
	For example, a proxy identified as 'ThisProxy' that receives a		For example, a proxy identified as 'ThisProxy' that receives a
	response bearing a header field:		response bearing a header field:
	Proxy-Status: SomeOtherProxy		Proxy-Status: SomeOtherProxy
	would add its own entry to the header field:		would add its own entry to the header field:
	Proxy-Status: SomeOtherProxy, ThisProxy		Proxy-Status: SomeOtherProxy, ThisProxy
	thus allowing it to append a trailer field:		thus allowing it to append a trailer field:
	Proxy-Status: ThisProxy; error=read_timeout		Proxy-Status: ThisProxy; error=read_timeout
			which would thereby allow a downstream recipient to understand that
	... which would thereby allow a downstream recipient to understand		processing by 'SomeOtherProxy' occurred before 'ThisProxy'.
	that processing by 'SomeOtherProxy' occurred before 'ThisProxy'.
	A client MAY promote the Proxy-Status trailer field into a header		A client MAY promote the Proxy-Status trailer field into a header
	field by following these steps:		field by following these steps:
	1. For each member trailer_member of the Proxy-Status trailer field		1. For each member trailer_member of the Proxy-Status trailer field
	value:		value:
	1. Let header_member be the first (left-most) value of the		1. Let header_member be the first (leftmost) value of the Proxy-
	Proxy-Status header field value, comparing the sf-token or		Status header field value, comparing the String or Token
	sf-string character-by-character and without consideration of		character by character without consideration of parameters.
	parameters.
	2. If no matching header_member is found, continue processing		2. If no matching header_member is found, continue processing
	the next trailer_member.		the next trailer_member.
	3. Replace header_member with trailer_member in its entirety,		3. Replace header_member with trailer_member in its entirety,
	including any parameters.		including any parameters.
	2. Remove the Proxy-Status trailer field, if empty.		2. Remove the Proxy-Status trailer field if empty.
	2.1. Proxy-Status Parameters		2.1. Proxy-Status Parameters
	This section lists parameters that can be used on the members of the		This section lists parameters that can be used on the members of the
	Proxy-Status field. Unrecognised parameters MUST be ignored.		Proxy-Status field. Unrecognised parameters MUST be ignored.
	2.1.1. error		2.1.1. error
	The "error" parameter's value is an sf-token that is a Proxy Error		The "error" parameter's value is a Token that is a proxy error type.
	Type. When present, it indicates that the intermediary encountered		When present, it indicates that the intermediary encountered an issue
	an issue when obtaining this response.		when obtaining this response.
	The presence of some Proxy Error Types indicates that the response		The presence of some proxy error types indicates that the response
	was generated by the intermediary itself, rather than being forwarded		was generated by the intermediary itself, rather than being forwarded
	from the origin server. This is the case when, for example, the		from the origin server. This is the case when, for example, the
	origin server can't be contacted, so the proxy has to create its own		origin server can't be contacted, so the proxy has to create its own
	response.		response.
	Other Proxy Error Types can be added to (potentially partial)		Other proxy error types can be added to (potentially partial)
	responses that were generated by the origin server or some other		responses that were generated by the origin server or some other
	inbound server. For example, if the forward connection abruptly		inbound server. For example, if the forward connection abruptly
	closes, an intermediary might add Proxy-Status with an appropriate		closes, an intermediary might add Proxy-Status with an appropriate
	error as a trailer field.		error as a trailer field.
	Proxy Error Types that are registered with a 'Response only generated		Proxy error types that are registered with a 'Response only generated
	by intermediaries' value of 'true' indicate that they can only occur		by intermediaries' value of 'true' indicate that they can only occur
	on responses generated by the intermediary. If the value is 'false',		in responses generated by the intermediary. If the value is 'false',
	the response might be generated by the intermediary or an inbound		the response might be generated by the intermediary or an inbound
	server.		server.
	Section 2.3 lists the Proxy Error Types defined in this document; new		Section 2.3 lists the proxy error types defined in this document; new
	ones can be defined using the procedure outlined in Section 2.4.		ones can be defined using the procedure outlined in Section 2.4.
	For example:		For example:
	HTTP/1.1 504 Gateway Timeout		HTTP/1.1 504 Gateway Timeout
	Proxy-Status: ExampleCDN; error=connection_timeout		Proxy-Status: ExampleCDN; error=connection_timeout
	indicates that this 504 response was generated by ExampleCDN, due to		indicates that this 504 response was generated by ExampleCDN due to a
	a connection timeout when going forward.		connection timeout when going forward.
	Or:		Or:
	HTTP/1.1 429 Too Many Requests		HTTP/1.1 429 Too Many Requests
	Proxy-Status: r34.example.net; error=http_request_error, ExampleCDN		Proxy-Status: r34.example.net; error=http_request_error, ExampleCDN
	indicates that this 429 Too Many Requests response was generated by		indicates that this 429 (Too Many Requests) response was generated by
	r34.example.net, not the CDN or the origin.		r34.example.net, not the CDN or the origin.
	When sending the error parameter, the most specific Proxy Error Type		When sending the error parameter, the most specific proxy error type
	SHOULD be sent, provided that it accurately represents the error		SHOULD be sent, provided that it accurately represents the error
	condition. If an appropriate Proxy Error Type is not defined, there		condition. If an appropriate proxy error type is not defined, there
	are a number of generic error types (e.g., proxy_internal_error,		are a number of generic error types (e.g., proxy_internal_error,
	http_protocol_error) that can be used. If they are not suitable,		http_protocol_error) that can be used. If they are not suitable,
	consider registering a new Proxy Error Type (see Section 2.4).		consider registering a new proxy error type (see Section 2.4).
	Each Proxy Error Type has a Recommended HTTP Status Code. When		Each proxy error type has a recommended HTTP status code. When
	generating a HTTP response containing "error", its HTTP status code		generating an HTTP response containing the "error", its HTTP status
	SHOULD be set to the Recommended HTTP Status Code. However, there		code SHOULD be set to the recommended HTTP status code. However,
	may be circumstances (e.g., for backwards compatibility with previous		there may be circumstances (e.g., for backwards compatibility with
	behaviours, a status code has already been sent) when another status		previous behaviours, a status code has already been sent) when
	code might be used.		another status code might be used.
	Proxy Error Types can also define any number of extra parameters for		Proxy error types can also define any number of extra parameters for
	use with that type. Their use, like all parameters, is optional. As		use with that type. Their use, like all parameters, is optional. As
	a result, if an extra parameter is used with a Proxy Error Type for		a result, if an extra parameter is used with a proxy error type for
	which it is not defined, it will be ignored.		which it is not defined, it will be ignored.
	2.1.2. next-hop		2.1.2. next-hop
	The "next-hop" parameter's value is an sf-string or sf-token that		The "next-hop" parameter's value is a String or Token that identifies
	identifies the intermediary or origin server selected (and used, if		the intermediary or origin server selected (and used, if contacted)
	contacted) to obtain this response. It might be a hostname, IP		to obtain this response. It might be a hostname, IP address, or
	address, or alias.		alias.
	For example:		For example:
	Proxy-Status: cdn.example.org; next-hop=backend.example.org:8001		Proxy-Status: cdn.example.org; next-hop=backend.example.org:8001
	indicates that cdn.example.org used backend.example.org:8001 as the		indicates that cdn.example.org used backend.example.org:8001 as the
	next hop for this request.		next hop for this request.
	2.1.3. next-protocol		2.1.3. next-protocol
	The "next-protocol" parameter's value indicates the ALPN protocol		The "next-protocol" parameter's value indicates the Application-Layer
	identifier [RFC7301] of the protocol used by the intermediary to		Protocol Negotiation (ALPN) protocol identifier [RFC7301] of the
	connect to the next hop when obtaining this response.		protocol used by the intermediary to connect to the next hop when
			obtaining this response.
	The value MUST be either an sf-token or sf-binary, representing a TLS		The value MUST be either a Token or Byte Sequence representing a TLS
	Application-Layer Protocol Negotiation (ALPN) Protocol ID (see		ALPN Protocol ID (see <https://www.iana.org/assignments/tls-
	https://www.iana.org/assignments/tls-extensiontype-values/tls-		extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-
	extensiontype-values.xhtml#alpn-protocol-ids [4]). If the protocol		ids>). If the protocol identifier is able to be expressed as a Token
	identifier is able to be expressed as an sf-token using ASCII		using ASCII encoding, that form MUST be used.
	encoding, that form MUST be used.
	For example:		For example:
	Proxy-Status: "proxy.example.org"; next-protocol=h2		Proxy-Status: "proxy.example.org"; next-protocol=h2
	Note that the APLN identifier is being used here to identify the		Note that the ALPN identifier is being used here to identify the
	protocol in use; it may or may not have been actually used in the		protocol in use; it may or may not have been actually used in the
	protocol negotiation.		protocol negotiation.
	2.1.4. received-status		2.1.4. received-status
	The "received-status" parameter's value indicates the HTTP status		The "received-status" parameter's value indicates the HTTP status
	code that the intermediary received from the next hop server when		code that the intermediary received from the next-hop server when
	obtaining this response.		obtaining this response.
	The value MUST be an sf-integer.		The value MUST be an Integer.
	For example:		For example:
	Proxy-Status: ExampleCDN; received-status=200		Proxy-Status: ExampleCDN; received-status=200
	2.1.5. details		2.1.5. details
	The "details" parameter's value is an sf-string containing additional		The "details" parameter's value is a String containing additional
	information not captured anywhere else. This can include		information not captured anywhere else. This can include
	implementation-specific or deployment-specific information.		implementation-specific or deployment-specific information.
	For example:		For example:
	Proxy-Status: proxy.example.net; error="http_protocol_error";		Proxy-Status: proxy.example.net; error="http_protocol_error";
	details="Malformed response header: space before colon"		details="Malformed response header: space before colon"
	2.2. Defining New Proxy-Status Parameters		2.2. Defining New Proxy-Status Parameters
	New Proxy-Status Parameters can be defined by registering them in the		New Proxy-Status parameters can be defined by registering them in the
	HTTP Proxy-Status Parameters registry.		"HTTP Proxy-Status Parameters" registry.
	Registration requests are reviewed and approved by Expert Review, as		Registration requests are reviewed and approved by Expert Review, per
	per [RFC8126], Section 4.5. A specification document is appreciated,		[RFC8126], Section 4.5. A specification document is appreciated but
	but not required.		not required.
	The Expert(s) should consider the following factors when evaluating		The expert(s) should consider the following factors when evaluating
	requests:		requests:
	o Community feedback		o Community feedback
	o If the value is sufficiently well-defined		o If the value is sufficiently well defined
	o Generic parameters are preferred over vendor-specific,		o Generic parameters are preferred over vendor-specific,
	application-specific or deployment-specific values. If a generic		application-specific, or deployment-specific values. If a generic
	value cannot be agreed upon in the community, the parameter's name		value cannot be agreed upon in the community, the parameter's name
	should be correspondingly specific (e.g., with a prefix that		should be correspondingly specific (e.g., with a prefix that
	identifies the vendor, application or deployment).		identifies the vendor, application, or deployment).
	o Parameter names should not conflict with registered extra		o Parameter names should not conflict with registered extra
	parameters in the Proxy Error Type Registry.		parameters in the "HTTP Proxy Error Types" registry.
	Registration requests should use the following template:		Registration requests should use the following template:
	o Name: [a name for the Proxy-Status Parameter that matches key]		Name: [a name for the Proxy-Status parameter that matches key]
	o Description: [a description of the parameter semantics and value]		Description: [a description of the parameter semantics and value]
	o Reference: [to a specification defining this parameter; optional]		Reference: [to a specification defining this parameter; optional]
	See the registry at https://iana.org/assignments/http-proxy-status		See the registry at <https://www.iana.org/assignments/http-proxy-
	[5] for details on where to send registration requests.		status> for details on where to send registration requests.
	2.3. Proxy Error Types		2.3. Proxy Error Types
	This section lists the Proxy Error Types defined by this document.		This section lists the proxy error types defined by this document.
	See Section 2.4 for information about defining new Proxy Error Types.		See Section 2.4 for information about defining new proxy error types.
	Note that implementations might not produce all Proxy Error Types.		Note that implementations might not produce all proxy error types.
	The set of types below is designed to map to existing states in		The set of types below is designed to map to existing states in
	implementations, and so may not be applicable to some.		implementations and therefore may not be applicable to some.
	2.3.1. DNS Timeout		2.3.1. DNS Timeout
	o Name: dns_timeout		Name: dns_timeout
	o Description: The intermediary encountered a timeout when trying to		Description: The intermediary encountered a timeout when trying to
	find an IP address for the next hop hostname.		find an IP address for the next-hop hostname.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 504		Recommended HTTP Status Code: 504
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.2. DNS Error		2.3.2. DNS Error
	o Name: dns_error		Name: dns_error
	o Description: The intermediary encountered a DNS error when trying		Description: The intermediary encountered a DNS error when trying to
	to find an IP address for the next hop hostname.		find an IP address for the next-hop hostname.
	o Extra Parameters:		Extra Parameters:
	* rcode: A sf-string conveying the DNS RCODE that indicates the		rcode: A String conveying the DNS RCODE that indicates the error
	error type. See [RFC8499], Section 3.		type. See [RFC8499], Section 3.
	* info-code: A sf-integer conveying the Extended DNS Error Code		info-code: An Integer conveying the Extended DNS Error Code INFO-
	info-code. See [RFC8914].		CODE. See [RFC8914].
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.3. Destination Not Found		2.3.3. Destination Not Found
	o Name: destination_not_found		Name: destination_not_found
	o Description: The intermediary cannot determine the appropriate		Description: The intermediary cannot determine the appropriate next
	next hop to use for this request; for example, it may not be		hop to use for this request; for example, it may not be
	configured. Note that this error is specific to gateways, which		configured. Note that this error is specific to gateways, which
	typically require specific configuration to identify the "backend"		typically require specific configuration to identify the "backend"
	server; forward proxies use in-band information to identify the		server; forward proxies use in-band information to identify the
	origin server.		origin server.
	o Extra Parameters: None.		Extra Parameters: None
			Recommended HTTP Status Code: 500
	o Recommended HTTP status code: 500
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.4. Destination Unavailable		2.3.4. Destination Unavailable
	o Name: destination_unavailable		Name: destination_unavailable
	o Description: The intermediary considers the next hop to be		Description: The intermediary considers the next hop to be
	unavailable; e.g., recent attempts to communicate with it may have		unavailable; e.g., recent attempts to communicate with it may have
	failed, or a health check may indicate that it is down.		failed, or a health check may indicate that it is down.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 503		Recommended HTTP Status Code: 503
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.5. Destination IP Prohibited		2.3.5. Destination IP Prohibited
	o Name: destination_ip_prohibited		Name: destination_ip_prohibited
	o Description: The intermediary is configured to prohibit		Description: The intermediary is configured to prohibit connections
	connections to the next hop IP address.		to the next-hop IP address.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]
	2.3.6. Destination IP Unroutable		Reference: RFC 9209
	o Name: destination_ip_unroutable		2.3.6. Destination IP Unroutable
	o Description: The intermediary cannot find a route to the next hop		Name: destination_ip_unroutable
	IP address.
	o Extra Parameters: None.		Description: The intermediary cannot find a route to the next-hop IP
			address.
	o Recommended HTTP status code: 502		Extra Parameters: None
	o Response only generated by intermediaries: true		Recommended HTTP Status Code: 502
			Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.7. Connection Refused		2.3.7. Connection Refused
	o Name: connection_refused		Name: connection_refused
	o Description: The intermediary's connection to the next hop was		Description: The intermediary's connection to the next hop was
	refused.		refused.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.8. Connection Terminated		2.3.8. Connection Terminated
	o Name: connection_terminated		Name: connection_terminated
	o Description: The intermediary's connection to the next hop was		Description: The intermediary's connection to the next hop was
	closed before complete response was received.		closed before a complete response was received.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.9. Connection Timeout		2.3.9. Connection Timeout
	o Name: connection_timeout		Name: connection_timeout
	o Description: The intermediary's attempt to open a connection to		Description: The intermediary's attempt to open a connection to the
	the next hop timed out.		next hop timed out.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 504		Recommended HTTP Status Code: 504
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.10. Connection Read Timeout		2.3.10. Connection Read Timeout
	o Name: connection_read_timeout		Name: connection_read_timeout
	o Description: The intermediary was expecting data on a connection		Description: The intermediary was expecting data on a connection
	(e.g., part of a response), but did not receive any new data in a		(e.g., part of a response) but did not receive any new data in a
	configured time limit.		configured time limit.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 504		Recommended HTTP Status Code: 504
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.11. Connection Write Timeout		2.3.11. Connection Write Timeout
	o Name: connection_write_timeout		Name: connection_write_timeout
	o Description: The intermediary was attempting to write data to a		Description: The intermediary was attempting to write data to a
	connection, but was not able to (e.g., because its buffers were		connection but was not able to (e.g., because its buffers were
	full).		full).
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 504		Recommended HTTP Status Code: 504
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.12. Connection Limit Reached		2.3.12. Connection Limit Reached
	o Name: connection_limit_reached		Name: connection_limit_reached
	o Description: The intermediary is configured to limit the number of		Description: The intermediary is configured to limit the number of
	connections it has to the next hop, and that limit has been		connections it has to the next hop, and that limit has been
	passed.		exceeded.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 503		Recommended HTTP Status Code: 503
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.13. TLS Protocol Error		2.3.13. TLS Protocol Error
	o Name: tls_protocol_error		Name: tls_protocol_error
	o Description: The intermediary encountered a TLS error when		Description: The intermediary encountered a TLS error when
	communicating with the next hop, either during handshake or		communicating with the next hop, either during the handshake or
	afterwards.		afterwards.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	o Notes: Not appropriate when a TLS alert is received; see		Notes: Not appropriate when a TLS alert is received; see
	tls_alert_received		tls_alert_received.
	2.3.14. TLS Certificate Error		2.3.14. TLS Certificate Error
	o Name: tls_certificate_error		Name: tls_certificate_error
	o Description: The intermediary encountered an error when verifying		Description: The intermediary encountered an error when verifying
	the certificate presented by the next hop.		the certificate presented by the next hop.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]
			Reference: RFC 9209
	2.3.15. TLS Alert Received		2.3.15. TLS Alert Received
	o Name: tls_alert_received		Name: tls_alert_received
	o Description: The intermediary received a TLS alert from the next		Description: The intermediary received a TLS alert from the next
	hop.		hop.
	o Extra Parameters:		Extra Parameters:
	* alert-id: an sf-integer containing the applicable value from		alert-id: An Integer containing the applicable value from the
	the TLS Alerts registry. See {!RFC8446}}.		"TLS Alerts" registry. See [RFC8446].
	* alert-message: an sf-token or sf-string containing the		alert-message: A Token or String containing the applicable
	applicable description string from the TLS Alerts registry.		description string from the "TLS Alerts" registry. See
	See [RFC8446].		[RFC8446].
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.16. HTTP Request Error		2.3.16. HTTP Request Error
	o Name: http_request_error		Name: http_request_error
	o Description: The intermediary is generating a client (4xx)		Description: The intermediary is generating a client (4xx) response
	response on the origin's behalf. Applicable status codes include		on the origin's behalf. Applicable status codes include (but are
	(but are not limited to) 400, 403, 405, 406, 408, 411, 413, 414,		not limited to) 400, 403, 405, 406, 408, 411, 413, 414, 415, 416,
	415, 416, 417, 429.		417, and 429.
	o Extra Parameters:		Extra Parameters:
	* status-code: an sf-integer containing the generated status		status-code: An Integer containing the generated status code.
	code.
	* status-phrase: an sf-string containing the generated status		status-phrase: A String containing the generated status phrase.
	phrase.
	o Recommended HTTP status code: The applicable 4xx status code		Recommended HTTP Status Code: The applicable 4xx status code
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	o Notes: This type helps distinguish between responses generated by
			Notes: This type helps distinguish between responses generated by
	intermediaries from those generated by the origin.		intermediaries from those generated by the origin.
	2.3.17. HTTP Request Denied		2.3.17. HTTP Request Denied
	o Name: http_request_denied		Name: http_request_denied
	o Description: The intermediary rejected the HTTP request based on		Description: The intermediary rejected the HTTP request based on its
	its configuration and/or policy settings. The request wasn't		configuration and/or policy settings. The request wasn't
	forwarded to the next hop.		forwarded to the next hop.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 403
	o Response only generated by intermediaries: true		Recommended HTTP Status Code: 403
	o Reference: [this document]		Response Only Generated by Intermediaries: true
			Reference: RFC 9209
	2.3.18. HTTP Incomplete Response		2.3.18. HTTP Incomplete Response
	o Name: http_response_incomplete		Name: http_response_incomplete
	o Description: The intermediary received an incomplete response to		Description: The intermediary received an incomplete response to the
	the request from the next hop.		request from the next hop.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.19. HTTP Response Header Section Too Large		2.3.19. HTTP Response Header Section Too Large
	o Name: http_response_header_section_size		Name: http_response_header_section_size
	o Description: The intermediary received a response to the request		Description: The intermediary received a response to the request
	whose header section was considered too large.		whose header section was considered too large.
	o Extra Parameters:		Extra Parameters:
	* header-section-size: an sf-integer indicating how large the		header-section-size: An Integer indicating how large the received
	headers received were. Note that they might not be complete;		headers were. Note that they might not be complete; i.e., the
	i.e., the intermediary may have discarded or refused additional		intermediary may have discarded or refused additional data.
	data.
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.20. HTTP Response Header Field Line Too Large		2.3.20. HTTP Response Header Field Line Too Large
	o Name: http_response_header_size		Name: http_response_header_size
	o Description: The intermediary received a response to the request		Description: The intermediary received a response to the request
	containing an individual header field line that was considered too		containing an individual header field line that was considered too
	large.		large.
	o Extra Parameters:		Extra Parameters:
	* header-name: an sf-string indicating the name of the header		header-name: A String indicating the name of the header field
	field that triggered the error.		that triggered the error.
	* header-size: an sf-integer indicating the size of the header		header-size: An Integer indicating the size of the header field
	field that triggered the error.		that triggered the error.
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.21. HTTP Response Body Too Large		2.3.21. HTTP Response Body Too Large
	o Name: http_response_body_size		Name: http_response_body_size
	o Description: The intermediary received a response to the request		Description: The intermediary received a response to the request
	whose body was considered too large.		whose body was considered too large.
	o Extra Parameters:		Extra Parameters:
	* body-size: an sf-integer indicating how large the body received		body-size: An Integer indicating how large the received body was.
	was. Note that it may not have been complete; i.e., the		Note that it may not have been complete; i.e., the intermediary
	intermediary may have discarded or refused additional data.		may have discarded or refused additional data.
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.22. HTTP Response Trailer Section Too Large		2.3.22. HTTP Response Trailer Section Too Large
	o Name: http_response_trailer_section_size		Name: http_response_trailer_section_size
	o Description: The intermediary received a response to the request		Description: The intermediary received a response to the request
	whose trailer section was considered too large.		whose trailer section was considered too large.
	o Extra Parameters:		Extra Parameters:
	* trailer-section-size: an sf-integer indicating how large the		trailer-section-size: An Integer indicating how large the
	trailers received were. Note that they might not be complete;		received trailers were. Note that they might not be complete;
	i.e., the intermediary may have discarded or refused additional		i.e., the intermediary may have discarded or refused additional
	data.		data.
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.23. HTTP Response Trailer Field Line Too Large		2.3.23. HTTP Response Trailer Field Line Too Large
	o Name: http_response_trailer_size		Name: http_response_trailer_size
	o Description: The intermediary received a response to the request		Description: The intermediary received a response to the request
	containing an individual trailer field line that was considered		containing an individual trailer field line that was considered
	too large.		too large.
	o Extra Parameters:		Extra Parameters:
	* trailer-name: an sf-string indicating the name of the trailer		trailer-name: A String indicating the name of the trailer field
	field that triggered the error.		that triggered the error.
	* trailer-size: an sf-integer indicating the size of the trailer		trailer-size: An Integer indicating the size of the trailer field
	field that triggered the error.		that triggered the error.
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.24. HTTP Response Transfer-Coding Error		2.3.24. HTTP Response Transfer-Coding Error
	o Name: http_response_transfer_coding		Name: http_response_transfer_coding
	o Description: The intermediary encountered an error decoding the		Description: The intermediary encountered an error decoding the
	transfer-coding of the response.		transfer coding of the response.
	o Extra Parameters:		Extra Parameters:
	* coding: an sf-token containing the specific coding (from the		coding: A Token containing the specific coding (from the "HTTP
	HTTP Transfer Coding Registry) that caused the error.		Transfer Coding Registry") that caused the error.
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.25. HTTP Response Content-Coding Error		2.3.25. HTTP Response Content-Coding Error
	o Name: http_response_content_coding		Name: http_response_content_coding
	o Description: The intermediary encountered an error decoding the		Description: The intermediary encountered an error decoding the
	content-coding of the response.		content coding of the response.
	o Extra Parameters:		Extra Parameters:
	* coding: an sf-token containing the specific coding (from the		coding: A Token containing the specific coding (from the "HTTP
	HTTP Content Coding Registry) that caused the error.		Content Coding Registry") that caused the error.
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.26. HTTP Response Timeout		2.3.26. HTTP Response Timeout
	o Name: http_response_timeout		Name: http_response_timeout
	o Description: The intermediary reached a configured time limit		Description: The intermediary reached a configured time limit
	waiting for the complete response.		waiting for the complete response.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 504		Recommended HTTP Status Code: 504
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.27. HTTP Upgrade Failed		2.3.27. HTTP Upgrade Failed
	o Name: http_upgrade_failed		Name: http_upgrade_failed
	o Description: The HTTP Upgrade between the intermediary and the		Description: The process of negotiating an upgrade of the HTTP
	next hop failed.		version between the intermediary and the next hop failed.
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.28. HTTP Protocol Error		2.3.28. HTTP Protocol Error
	o Name: http_protocol_error		Name: http_protocol_error
	o Description: The intermediary encountered a HTTP protocol error		Description: The intermediary encountered an HTTP protocol error
	when communicating with the next hop. This error should only be		when communicating with the next hop. This error should only be
	used when a more specific one is not defined.		used when a more specific one is not defined.
	o Extra Parameters: None.		Extra Parameters: None
			Recommended HTTP Status Code: 502
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false		Response Only Generated by Intermediaries: false
	o Reference: [this document]		Reference: RFC 9209
	2.3.29. Proxy Internal Response		2.3.29. Proxy Internal Response
	o Name: proxy_internal_response		Name: proxy_internal_response
	o Description: The intermediary generated the response locally,		Description: The intermediary generated the response itself without
	without attempting to connect to the next hop (e.g. in response to		attempting to connect to the next hop.
	a request to a debug endpoint terminated at the intermediary).
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: The most appropriate status code for		Recommended HTTP Status Code: The most appropriate status code for
	the response		the response
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.30. Proxy Internal Error		2.3.30. Proxy Internal Error
	o Name: proxy_internal_error		Name: proxy_internal_error
	o Description: The intermediary encountered an internal error		Description: The intermediary encountered an internal error
	unrelated to the origin.		unrelated to the origin.
	o Extra Parameters: None		Extra Parameters: None
	o Recommended HTTP status code: 500		Recommended HTTP Status Code: 500
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.31. Proxy Configuration Error		2.3.31. Proxy Configuration Error
	o Name: proxy_configuration_error		Name: proxy_configuration_error
	o Description: The intermediary encountered an error regarding its		Description: The intermediary encountered an error regarding its
	configuration.		configuration.
	o Extra Parameters: None		Extra Parameters: None
	o Recommended HTTP status code: 500
	o Response only generated by intermediaries: true		Recommended HTTP Status Code: 500
			Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.3.32. Proxy Loop Detected		2.3.32. Proxy Loop Detected
	o Name: proxy_loop_detected		Name: proxy_loop_detected
	o Description: The intermediary tried to forward the request to		Description: The intermediary tried to forward the request to
	itself, or a loop has been detected using different means (e.g.		itself, or a loop has been detected using different means (e.g.,
	[RFC8586]).		[RFC8586]).
	o Extra Parameters: None.		Extra Parameters: None
	o Recommended HTTP status code: 502		Recommended HTTP Status Code: 502
	o Response only generated by intermediaries: true		Response Only Generated by Intermediaries: true
	o Reference: [this document]		Reference: RFC 9209
	2.4. Defining New Proxy Error Types		2.4. Defining New Proxy Error Types
	New Proxy Error Types can be defined by registering them in the HTTP		New proxy error types can be defined by registering them in the "HTTP
	Proxy Error Types registry.		Proxy Error Types" registry.
	Registration requests are reviewed and approved by Expert Review, as		Registration requests are reviewed and approved by Expert Review, per
	per [RFC8126], Section 4.5. A specification document is appreciated,		[RFC8126], Section 4.5. A specification document is appreciated but
	but not required.		not required.
	The Expert(s) should consider the following factors when evaluating		The expert(s) should consider the following factors when evaluating
	requests:		requests:
	o Community feedback		o Community feedback
	o If the value is sufficiently well-defined		o If the value is sufficiently well-defined
	o Generic types are preferred over vendor-specific, application-		o Generic types are preferred over vendor-specific, application-
	specific or deployment-specific values. If a generic value cannot		specific, or deployment-specific values. If a generic value
	be agreed upon in the community, the types's name should be		cannot be agreed upon in the community, the type's name should be
	correspondingly specific (e.g., with a prefix that identifies the		correspondingly specific (e.g., with a prefix that identifies the
	vendor, application or deployment).		vendor, application, or deployment).
	o Extra Parameters should not conflict with registered Proxy-Status		o Extra parameters should not conflict with registered Proxy-Status
	parameters.		parameters.
	Registration requests should use the following template:		Registration requests should use the following template:
	o Name: [a name for the Proxy Error Type that matches sf-token]		Name: [a name for the proxy error type that is of type Token ]
			Description: [a description of the conditions that generate the
	o Description: [a description of the conditions that generate the		proxy error type]
	Proxy Error Type]
	o Extra Parameters: [zero or more optional parameters, along with		Extra Parameters: [zero or more optional parameters, along with
	their allowable type(s)]		their allowable Structured Type(s)]
	o Recommended HTTP status code: [the appropriate HTTP status code		Recommended HTTP Status Code: [the appropriate HTTP status code for
	for this entry]		this entry]
	o Response only generated by intermediaries: ['true' or 'false']		Response Only Generated by Intermediaries: ['true' or 'false']
	o Reference: [to a specification defining this error type; optional]		Reference: [to a specification defining this error type; optional]
	o Notes: [optional]		Notes: [optional]
	If the Proxy Error Type might occur in responses that are not		If the proxy error type might occur in responses that are not
	generated by the intermediary -- for example, when an error is		generated by the intermediary -- for example, when an error is
	detected as the response is streamed from a forward connection,		detected as the response is streamed from a forward connection,
	causing a Proxy-Status trailer field to be appended -- the 'Response		causing a Proxy-Status trailer field to be appended -- the 'Response
	only generated by intermediaries' should be 'false'. If the Proxy		only generated by intermediaries' should be 'false'. If the proxy
	Error Type only occurs in responses that are generated by the		error type only occurs in responses that are generated by the
	intermediary, it should be 'true'.		intermediary, it should be 'true'.
	See the registry at https://iana.org/assignments/http-proxy-status		See the registry at <https://www.iana.org/assignments/http-proxy-
	[6] for details on where to send registration requests.		status> for details on where to send registration requests.
	3. IANA Considerations		3. IANA Considerations
	Upon publication, please create the HTTP Proxy-Status Parameters		IANA has created the "HTTP Proxy-Status Parameters" registry and the
	registry and the HTTP Proxy Error Types registry at		"HTTP Proxy Error Types" registry at
	https://iana.org/assignments/http-proxy-status [7] and populate them		<https://www.iana.org/assignments/http-proxy-status> and has
	with the types defined in Section 2.1 and Section 2.3 respectively;		populated them with the types defined in Sections 2.1 and 2.3
	see Section 2.2 and Section 2.4 for its associated procedures.		respectively; see Sections 2.2 and 2.4 for their associated
			procedures.
	Additionally, please register the following entry in the Hypertext
	Transfer Protocol (HTTP) Field Name Registry:
	o Field name: Proxy-Status		Additionally, the following entry has been added to the "Hypertext
			Transfer Protocol (HTTP) Field Name Registry":
	o Status: permanent		Field name: Proxy-Status
	o Specification document(s): [this document]		Status: permanent
	o Comments:		Specification document(s): RFC 9209
	4. Security Considerations		4. Security Considerations
	One of the primary security concerns when using Proxy-Status is		One of the primary security concerns when using Proxy-Status is
	leaking information that might aid an attacker. For example,		leaking information that might aid an attacker. For example,
	information about the intermediary's configuration and back-end		information about the intermediary's configuration and backend
	topology can be exposed, allowing attackers to directly target back-		topology can be exposed, allowing attackers to directly target
	end services that are not prepared for high traffic volume or		backend services that are not prepared for high traffic volume or
	malformed inputs. Some information might only be suitable to reveal		malformed inputs. Some information might only be suitable to reveal
	to authorized parties.		to authorized parties.
	As a result, care needs to be taken when deciding to generate a		As a result, care needs to be taken when deciding to generate a
	Proxy-Status field and what information to include in it. Note that		Proxy-Status field and what information to include in it. Note that
	intermediaries are not required to generate a Proxy-Status field in		intermediaries are not required to generate a Proxy-Status field in
	any response, and can conditionally generate them based upon request		any response and can conditionally generate them based upon request
	attributes (e.g., authentication tokens, IP address).		attributes (e.g., authentication tokens, IP address).
	Likewise, generation of all parameters is optional, as is generation		Likewise, generation of all parameters is optional, as is the
	of the field itself. Also, the field's content is not verified; an		generation of the field itself. Also, the field's content is not
	intermediary can claim certain actions (e.g., sending a request over		verified; an intermediary can claim certain actions (e.g., sending a
	an encrypted channel) but fail to actually do that.		request over an encrypted channel) but fail to actually do that.
	5. References		5. References
	5.1. Normative References		5.1. Normative References
	[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP		[HTTP] Fielding, R., Nottingham, M., and J. Reschke, "HTTP
	Semantics", draft-ietf-httpbis-semantics-19 (work in		Semantics", draft-ietf-httpbis-semantics-19 (work in
	progress), September 2021.		progress), September 2021.
	[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>.
	[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,		[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
	"Transport Layer Security (TLS) Application-Layer Protocol		"Transport Layer Security (TLS) Application-Layer Protocol
	skipping to change at page 23, line 37 ¶		skipping to change at page 23, line 10 ¶
	[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>.
	[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol		[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
	Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,		Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
	<https://www.rfc-editor.org/info/rfc8446>.		<https://www.rfc-editor.org/info/rfc8446>.
	[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS		[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
	Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,		Terminology", RFC 8499, DOI 10.17487/RFC8499, January
	January 2019, <https://www.rfc-editor.org/info/rfc8499>.		2019, <https://www.rfc-editor.org/info/rfc8499>.
	[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.		[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
	Lawrence, "Extended DNS Errors", RFC 8914,		Lawrence, "Extended DNS Errors", RFC 8914,
	DOI 10.17487/RFC8914, October 2020,		DOI 10.17487/RFC8914, October 2020,
	<https://www.rfc-editor.org/info/rfc8914>.		<https://www.rfc-editor.org/info/rfc8914>.
	[STRUCTURED-FIELDS]		[STRUCTURED-FIELDS]
	Nottingham, M. and P-H. Kamp, "Structured Field Values for		Nottingham, M. and P. Kamp, "Structured Field Values for
	HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,		HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
	<https://www.rfc-editor.org/info/rfc8941>.		<https://www.rfc-editor.org/info/rfc8941>.
	5.2. Informative References		5.2. Informative References
	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF", STD 68, RFC 5234,
	DOI 10.17487/RFC5234, January 2008,
	<https://www.rfc-editor.org/info/rfc5234>.
	[RFC8586] Ludin, S., Nottingham, M., and N. Sullivan, "Loop		[RFC8586] Ludin, S., Nottingham, M., and N. Sullivan, "Loop
	Detection in Content Delivery Networks (CDNs)", RFC 8586,		Detection in Content Delivery Networks (CDNs)", RFC 8586,
	DOI 10.17487/RFC8586, April 2019,		DOI 10.17487/RFC8586, April 2019,
	<https://www.rfc-editor.org/info/rfc8586>.		<https://www.rfc-editor.org/info/rfc8586>.
	5.3. URIs
	[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
	[2] https://httpwg.org/
	[3] https://github.com/httpwg/http-extensions/labels/proxy-status
	[4] https://www.iana.org/assignments/tls-extensiontype-values/tls-
	extensiontype-values.xhtml#alpn-protocol-ids
	[5] https://iana.org/assignments/http-proxy-status
	[6] https://iana.org/assignments/http-proxy-status
	[7] https://iana.org/assignments/http-proxy-status
	Authors' Addresses		Authors' Addresses
	Mark Nottingham		Mark Nottingham
	Fastly		Fastly
	Prahran		Prahran
	Australia		Australia
	Email: mnot@mnot.net		Email: mnot@mnot.net
	URI: https://www.mnot.net/		URI: https://www.mnot.net/
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