draft-ietf-httpbis-proxy-status-08.txt		draft-ietf-httpbis-proxy-status-latest.txt
	HTTP Working Group M. Nottingham
	Internet-Draft Fastly
	Intended status: Standards Track P. Sikora
	Expires: April 16, 2022 Google
	October 13, 2021
	The Proxy-Status HTTP Response Header Field
	draft-ietf-httpbis-proxy-status-08
	Abstract
	This document defines the Proxy-Status HTTP field to convey the
	details of intermediary response handling, including generated
	errors.
	Note to Readers
	_RFC EDITOR: please remove this section before publication_
	Discussion of this draft takes place on the HTTP working group
	mailing list (ietf-http-wg@w3.org), which is archived at
	https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
	Working Group information can be found at https://httpwg.org/ [2];
	source code and issues list for this draft can be found at
	https://github.com/httpwg/http-extensions/labels/proxy-status [3].
	Status of This Memo
	This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering
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	This Internet-Draft will expire on April 16, 2022.
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	1. Introduction
	HTTP intermediaries (see Section 3.7 of [HTTP]) -- including both
	forward proxies and gateways (also known as "reverse proxies") --
	have become an increasingly significant part of HTTP deployments. In
	particular, reverse proxies and Content Delivery Networks (CDNs) form
	part of the critical infrastructure of many Web sites.
	Typically, HTTP intermediaries forward requests towards the origin
	server (inbound) and then forward their responses back to clients
	(outbound). However, if an error occurs before a response is
	obtained from an inbound server, the response is often generated by
	the intermediary itself.
	HTTP accommodates these types of errors with a few status codes; for
	example, 502 Bad Gateway and 504 Gateway Timeout. However,
	experience has shown that more information is necessary to aid
	debugging and communicate what's happened to the client.
	Additionally, intermediaries sometimes want to convey additional
	information about their handling of a response, even if they did not
	generate it.
	To enable these uses, Section 2 defines a new HTTP response field to
	allow intermediaries to convey details of their handling of a
	response. Section 2.1 enumerates the information that can be added
	to the field by intermediaries, which can be extended as per
	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;
	these can likewise be extended as per Section 2.4.
	1.1. Notational Conventions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.
	This specification uses Structured Fields [STRUCTURED-FIELDS] to
	specify syntax and parsing, and ABNF [RFC5234] as a shorthand for
	that syntax. The terms sf-list, sf-item, sf-string, sf-token, sf-
	integer and key refer to the structured types defined therein.
	Note that in this specification, "proxy" is used to indicate both
	forward and reverse proxies, otherwise known as gateways. "Next hop"
	indicates the connection in the direction leading to the origin
	server for the request.
	2. The Proxy-Status HTTP Field
	The Proxy-Status HTTP response field allows an intermediary to convey
	additional information about its handling of a response and its
	associated request. The syntax of this header field conforms to
	[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
	the response. The first member of the list represents the
	intermediary closest to the origin server, and the last member of the
	list represents the intermediary closest to the user agent.
	For example:
	Proxy-Status: revproxy1.example.net, ExampleCDN
	indicates that this response was handled first by
	revproxy1.example.net (a reverse proxy adjacent to the origin server)
	and then ExampleCDN.
	Intermediaries determine when it is appropriate to add the Proxy-
	Status field to a response. Some might decide to append to it to all
	responses, whereas others might only do so when specifically
	configured to, or when the request contains a header field that
	activates a debugging mode.
	Each member of the list identifies the intermediary that inserted the
	value, and MUST have a type of either sf-string or sf-token.
	Depending on the deployment, this might be a service name (but not a
	software or hardware product name; e.g., "Example CDN"is appropriate,
	but "ExampleProxy" is not, because it doesn't identify the
	deployment), a hostname ("proxy-3.example.com"), an IP address, or a
	generated string.
	Parameters on each member (as per Section 3.1.2 of
	[STRUCTURED-FIELDS]) convey additional information about that
	intermediary's handling of the response and its associated request;
	see Section 2.1. While all of these parameters are OPTIONAL,
	intermediaries are encouraged to provide as much information as
	possible (but see Section 4 for security considerations in doing so).
	When adding a value to the Proxy-Status field, intermediaries SHOULD
	preserve the existing members of the field to allow debugging of the
	entire chain of intermediaries handling the request, unless
	explicitly configured to remove them (e.g., to prevent internal
	network details from leaking; see Section 4).
	Origin servers MUST NOT generate the Proxy-Status field.
	Proxy-Status MAY be sent as a HTTP trailer field. For example, if an
	intermediary is streaming a response and the inbound connection
	suddenly terminates, Proxy-Status can only be appended to the trailer
	section of the outbound message, since the header section has already
	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
	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.
	To allow recipients to reconstruct the relative ordering of Proxy-
	Status members conveyed in trailer fields with those conveyed in
	header fields, an intermediary MUST NOT send Proxy-Status as a
	trailer field unless it has also generated a Proxy-Status header
	field with the same member (although potentially different
	parameters) in that message.
	For example, a proxy identified as 'ThisProxy' that receives a
	response bearing a header field:
	Proxy-Status: SomeOtherProxy
	would add its own entry to the header field:
	Proxy-Status: SomeOtherProxy, ThisProxy
	thus allowing it to append a trailer field:
	Proxy-Status: ThisProxy; error=read_timeout
	... which would thereby allow a downstream recipient to understand
	that processing by 'SomeOtherProxy' occurred before 'ThisProxy'.
	A client MAY promote the Proxy-Status trailer field into a header
	field by following these steps:
	1. For each member trailer_member of the Proxy-Status trailer field
	value:
	1. Let header_member be the first (left-most) value of the
	Proxy-Status header field value, comparing the sf-token or
	sf-string character-by-character and without consideration of
	parameters.
	2. If no matching header_member is found, continue processing
	the next trailer_member.
	3. Replace header_member with trailer_member in its entirety,
	including any parameters.
	2. Remove the Proxy-Status trailer field, if empty.
	2.1. Proxy-Status Parameters
	This section lists parameters that can be used on the members of the
	Proxy-Status field. Unrecognised parameters MUST be ignored.
	2.1.1. error
	The "error" parameter's value is an sf-token that is a Proxy Error
	Type. When present, it indicates that the intermediary encountered
	an issue when obtaining this response.
	The presence of some Proxy Error Types indicates that the response
	was generated by the intermediary itself, rather than being forwarded
	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
	response.
	Other Proxy Error Types can be added to (potentially partial)
	responses that were generated by the origin server or some other
	inbound server. For example, if the forward connection abruptly
	closes, an intermediary might add Proxy-Status with an appropriate
	error as a trailer field.
	Proxy Error Types that are registered with a 'Response only generated
	by intermediaries' value of 'true' indicate that they can only occur
	on responses generated by the intermediary. If the value is 'false',
	the response might be generated by the intermediary or an inbound
	server.
	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.
	For example:
	HTTP/1.1 504 Gateway Timeout
	Proxy-Status: ExampleCDN; error=connection_timeout
	indicates that this 504 response was generated by ExampleCDN, due to
	a connection timeout when going forward.
	Or:
	HTTP/1.1 429 Too Many Requests
	Proxy-Status: r34.example.net; error=http_request_error, ExampleCDN
	indicates that this 429 Too Many Requests response was generated by
	r34.example.net, not the CDN or the origin.
	When sending the error parameter, the most specific Proxy Error Type
	SHOULD be sent, provided that it accurately represents the error
	condition. If an appropriate Proxy Error Type is not defined, there
	are a number of generic error types (e.g., proxy_internal_error,
	http_protocol_error) that can be used. If they are not suitable,
	consider registering a new Proxy Error Type (see Section 2.4).
	Each Proxy Error Type has a Recommended HTTP Status Code. When
	generating a HTTP response containing "error", its HTTP status code
	SHOULD be set to the Recommended HTTP Status Code. However, there
	may be circumstances (e.g., for backwards compatibility with previous
	behaviours, a status code has already been sent) when another status
	code might be used.
	Proxy Error Types can also define any number of extra parameters for
	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
	which it is not defined, it will be ignored.
	2.1.2. next-hop
	The "next-hop" parameter's value is an sf-string or sf-token that
	identifies the intermediary or origin server selected (and used, if
	contacted) to obtain this response. It might be a hostname, IP
	address, or alias.
	For example:
	Proxy-Status: cdn.example.org; next-hop=backend.example.org:8001
	indicates that cdn.example.org used backend.example.org:8001 as the
	next hop for this request.
	2.1.3. next-protocol
	The "next-protocol" parameter's value indicates the ALPN protocol
	identifier [RFC7301] of the 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
	Application-Layer Protocol Negotiation (ALPN) Protocol ID (see
	https://www.iana.org/assignments/tls-extensiontype-values/tls-
	extensiontype-values.xhtml#alpn-protocol-ids [4]). If the protocol
	identifier is able to be expressed as an sf-token using ASCII
	encoding, that form MUST be used.
	For example:
	Proxy-Status: "proxy.example.org"; next-protocol=h2
	Note that the APLN identifier is being used here to identify the
	protocol in use; it may or may not have been actually used in the
	protocol negotiation.
	2.1.4. received-status
	The "received-status" parameter's value indicates the HTTP status
	code that the intermediary received from the next hop server when
	obtaining this response.
	The value MUST be an sf-integer.
	For example:
	Proxy-Status: ExampleCDN; received-status=200
	2.1.5. details
	The "details" parameter's value is an sf-string containing additional
	information not captured anywhere else. This can include
	implementation-specific or deployment-specific information.
	For example:
	Proxy-Status: proxy.example.net; error="http_protocol_error";
	details="Malformed response header: space before colon"
	2.2. Defining New Proxy-Status Parameters
	New Proxy-Status Parameters can be defined by registering them in the
	HTTP Proxy-Status Parameters registry.
	Registration requests are reviewed and approved by Expert Review, as
	per [RFC8126], Section 4.5. A specification document is appreciated,
	but not required.
	The Expert(s) should consider the following factors when evaluating
	requests:
	o Community feedback
	o If the value is sufficiently well-defined
	o Generic parameters are preferred over vendor-specific,
	application-specific or deployment-specific values. If a generic
	value cannot be agreed upon in the community, the parameter's name
	should be correspondingly specific (e.g., with a prefix that
	identifies the vendor, application or deployment).
	o Parameter names should not conflict with registered extra
	parameters in the Proxy Error Type Registry.
	Registration requests should use the following template:
	o Name: [a name for the Proxy-Status Parameter that matches key]
	o Description: [a description of the parameter semantics and value]
	o Reference: [to a specification defining this parameter; optional]
	See the registry at https://iana.org/assignments/http-proxy-status
	[5] for details on where to send registration requests.
	2.3. Proxy Error Types
	This section lists the Proxy Error Types defined by this document.
	See Section 2.4 for information about defining new 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
	implementations, and so may not be applicable to some.
	2.3.1. DNS Timeout
	o Name: dns_timeout
	o Description: The intermediary encountered a timeout when trying to
	find an IP address for the next hop hostname.
	o Extra Parameters: None.
	o Recommended HTTP status code: 504
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.2. DNS Error
	o Name: dns_error
	o Description: The intermediary encountered a DNS error when trying
	to find an IP address for the next hop hostname.
	o Extra Parameters:
	* rcode: A sf-string conveying the DNS RCODE that indicates the
	error type. See [RFC8499], Section 3.
	* info-code: A sf-integer conveying the Extended DNS Error Code
	info-code. See [RFC8914].
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.3. Destination Not Found
	o Name: destination_not_found
	o Description: The intermediary cannot determine the appropriate
	next hop to use for this request; for example, it may not be
	configured. Note that this error is specific to gateways, which
	typically require specific configuration to identify the "backend"
	server; forward proxies use in-band information to identify the
	origin server.
	o Extra Parameters: None.
	o Recommended HTTP status code: 500
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.4. Destination Unavailable
	o Name: destination_unavailable
	o Description: The intermediary considers the next hop to be
	unavailable; e.g., recent attempts to communicate with it may have
	failed, or a health check may indicate that it is down.
	o Extra Parameters: None.
	o Recommended HTTP status code: 503
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.5. Destination IP Prohibited
	o Name: destination_ip_prohibited
	o Description: The intermediary is configured to prohibit
	connections to the next hop IP address.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.6. Destination IP Unroutable
	o Name: destination_ip_unroutable
	o Description: The intermediary cannot find a route to the next hop
	IP address.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.7. Connection Refused
	o Name: connection_refused
	o Description: The intermediary's connection to the next hop was
	refused.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.8. Connection Terminated
	o Name: connection_terminated
	o Description: The intermediary's connection to the next hop was
	closed before complete response was received.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.9. Connection Timeout
	o Name: connection_timeout
	o Description: The intermediary's attempt to open a connection to
	the next hop timed out.
	o Extra Parameters: None.
	o Recommended HTTP status code: 504
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.10. Connection Read Timeout
	o Name: connection_read_timeout
	o Description: The intermediary was expecting data on a connection
	(e.g., part of a response), but did not receive any new data in a
	configured time limit.
	o Extra Parameters: None.
	o Recommended HTTP status code: 504
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.11. Connection Write Timeout
	o Name: connection_write_timeout
	o Description: The intermediary was attempting to write data to a
	connection, but was not able to (e.g., because its buffers were
	full).
	o Extra Parameters: None.
	o Recommended HTTP status code: 504
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.12. Connection Limit Reached
	o Name: connection_limit_reached
	o Description: The intermediary is configured to limit the number of
	connections it has to the next hop, and that limit has been
	passed.
	o Extra Parameters: None.
	o Recommended HTTP status code: 503
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.13. TLS Protocol Error
	o Name: tls_protocol_error
	o Description: The intermediary encountered a TLS error when
	communicating with the next hop, either during handshake or
	afterwards.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	o Notes: Not appropriate when a TLS alert is received; see
	tls_alert_received
	2.3.14. TLS Certificate Error
	o Name: tls_certificate_error
	o Description: The intermediary encountered an error when verifying
	the certificate presented by the next hop.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.15. TLS Alert Received
	o Name: tls_alert_received
	o Description: The intermediary received a TLS alert from the next
	hop.
	o Extra Parameters:
	* alert-id: an sf-integer containing the applicable value from
	the TLS Alerts registry. See {!RFC8446}}.
	* alert-message: an sf-token or sf-string containing the
	applicable description string from the TLS Alerts registry.
	See [RFC8446].
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.16. HTTP Request Error
	o Name: http_request_error
	o Description: The intermediary is generating a client (4xx)
	response on the origin's behalf. Applicable status codes include
	(but are not limited to) 400, 403, 405, 406, 408, 411, 413, 414,
	415, 416, 417, 429.
	o Extra Parameters:
	* status-code: an sf-integer containing the generated status
	code.
	* status-phrase: an sf-string containing the generated status
	phrase.
	o Recommended HTTP status code: The applicable 4xx status code
	o Response only generated by intermediaries: true
	o Reference: [this document]
	o Notes: This type helps distinguish between responses generated by
	intermediaries from those generated by the origin.
	2.3.17. HTTP Request Denied
	o Name: http_request_denied
	o Description: The intermediary rejected the HTTP request based on
	its configuration and/or policy settings. The request wasn't
	forwarded to the next hop.
	o Extra Parameters: None.
	o Recommended HTTP status code: 403
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.18. HTTP Incomplete Response
	o Name: http_response_incomplete
	o Description: The intermediary received an incomplete response to
	the request from the next hop.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.19. HTTP Response Header Section Too Large
	o Name: http_response_header_section_size
	o Description: The intermediary received a response to the request
	whose header section was considered too large.
	o Extra Parameters:
	* header-section-size: an sf-integer indicating how large the
	headers received were. Note that they might not be complete;
	i.e., the intermediary may have discarded or refused additional
	data.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.20. HTTP Response Header Field Line Too Large
	o Name: http_response_header_size
	o Description: The intermediary received a response to the request
	containing an individual header field line that was considered too
	large.
	o Extra Parameters:
	* header-name: an sf-string indicating the name of the header
	field that triggered the error.
	* header-size: an sf-integer indicating the size of the header
	field that triggered the error.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.21. HTTP Response Body Too Large
	o Name: http_response_body_size
	o Description: The intermediary received a response to the request
	whose body was considered too large.
	o Extra Parameters:
	* body-size: an sf-integer indicating how large the body received
	was. Note that it may not have been complete; i.e., the
	intermediary may have discarded or refused additional data.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.22. HTTP Response Trailer Section Too Large
	o Name: http_response_trailer_section_size
	o Description: The intermediary received a response to the request
	whose trailer section was considered too large.
	o Extra Parameters:
	* trailer-section-size: an sf-integer indicating how large the
	trailers received were. Note that they might not be complete;
	i.e., the intermediary may have discarded or refused additional
	data.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.23. HTTP Response Trailer Field Line Too Large
	o Name: http_response_trailer_size
	o Description: The intermediary received a response to the request
	containing an individual trailer field line that was considered
	too large.
	o Extra Parameters:
	* trailer-name: an sf-string indicating the name of the trailer
	field that triggered the error.
	* trailer-size: an sf-integer indicating the size of the trailer
	field that triggered the error.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.24. HTTP Response Transfer-Coding Error
	o Name: http_response_transfer_coding
	o Description: The intermediary encountered an error decoding the
	transfer-coding of the response.
	o Extra Parameters:
	* coding: an sf-token containing the specific coding (from the
	HTTP Transfer Coding Registry) that caused the error.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.25. HTTP Response Content-Coding Error
	o Name: http_response_content_coding
	o Description: The intermediary encountered an error decoding the
	content-coding of the response.
	o Extra Parameters:
	* coding: an sf-token containing the specific coding (from the
	HTTP Content Coding Registry) that caused the error.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.26. HTTP Response Timeout
	o Name: http_response_timeout
	o Description: The intermediary reached a configured time limit
	waiting for the complete response.
	o Extra Parameters: None.
	o Recommended HTTP status code: 504
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.27. HTTP Upgrade Failed
	o Name: http_upgrade_failed
	o Description: The HTTP Upgrade between the intermediary and the
	next hop failed.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.28. HTTP Protocol Error
	o Name: http_protocol_error
	o Description: The intermediary encountered a HTTP protocol error
	when communicating with the next hop. This error should only be
	used when a more specific one is not defined.
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: false
	o Reference: [this document]
	2.3.29. Proxy Internal Response
	o Name: proxy_internal_response
	o Description: The intermediary generated the response locally,
	without attempting to connect to the next hop (e.g. in response to
	a request to a debug endpoint terminated at the intermediary).
	o Extra Parameters: None.
	o Recommended HTTP status code: The most appropriate status code for
	the response
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.30. Proxy Internal Error
	o Name: proxy_internal_error
	o Description: The intermediary encountered an internal error
	unrelated to the origin.
	o Extra Parameters: None
	o Recommended HTTP status code: 500
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.31. Proxy Configuration Error
	o Name: proxy_configuration_error
	o Description: The intermediary encountered an error regarding its
	configuration.
	o Extra Parameters: None
	o Recommended HTTP status code: 500
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.3.32. Proxy Loop Detected
	o Name: proxy_loop_detected
	o Description: The intermediary tried to forward the request to
	itself, or a loop has been detected using different means (e.g.
	[RFC8586]).
	o Extra Parameters: None.
	o Recommended HTTP status code: 502
	o Response only generated by intermediaries: true
	o Reference: [this document]
	2.4. Defining New Proxy Error Types
	New Proxy Error Types can be defined by registering them in the HTTP
	Proxy Error Types registry.
	Registration requests are reviewed and approved by Expert Review, as
	per [RFC8126], Section 4.5. A specification document is appreciated,
	but not required.
	The Expert(s) should consider the following factors when evaluating
	requests:
	o Community feedback
	o If the value is sufficiently well-defined
	o Generic types are preferred over vendor-specific, application-
	specific or deployment-specific values. If a generic value cannot
	be agreed upon in the community, the types's name should be
	correspondingly specific (e.g., with a prefix that identifies the
	vendor, application or deployment).
	o Extra Parameters should not conflict with registered Proxy-Status
	parameters.
	Registration requests should use the following template:
	o Name: [a name for the Proxy Error Type that matches sf-token]
	o Description: [a description of the conditions that generate the
	Proxy Error Type]
	o Extra Parameters: [zero or more optional parameters, along with
	their allowable type(s)]
	o Recommended HTTP status code: [the appropriate HTTP status code
	for this entry]
	o Response only generated by intermediaries: ['true' or 'false']
	o Reference: [to a specification defining this error type; optional]
	o Notes: [optional]
	If the Proxy Error Type might occur in responses that are not
	generated by the intermediary -- for example, when an error is
	detected as the response is streamed from a forward connection,
	causing a Proxy-Status trailer field to be appended -- the 'Response
	only generated by intermediaries' should be 'false'. If the Proxy
	Error Type only occurs in responses that are generated by the
	intermediary, it should be 'true'.
	See the registry at https://iana.org/assignments/http-proxy-status
	[6] for details on where to send registration requests.
	3. IANA Considerations
	Upon publication, please create the HTTP Proxy-Status Parameters
	registry and the HTTP Proxy Error Types registry at
	https://iana.org/assignments/http-proxy-status [7] and populate them
	with the types defined in Section 2.1 and Section 2.3 respectively;
	see Section 2.2 and Section 2.4 for its associated procedures.
	Additionally, please register the following entry in the Hypertext
	Transfer Protocol (HTTP) Field Name Registry:
	o Field name: Proxy-Status
	o Status: permanent
	o Specification document(s): [this document]
	o Comments:
	4. Security Considerations
	One of the primary security concerns when using Proxy-Status is
	leaking information that might aid an attacker. For example,
	information about the intermediary's configuration and back-end
	topology can be exposed, allowing attackers to directly target back-
	end services that are not prepared for high traffic volume or
	malformed inputs. Some information might only be suitable to reveal
	to authorized parties.
	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
	intermediaries are not required to generate a Proxy-Status field in
	any response, and can conditionally generate them based upon request
	attributes (e.g., authentication tokens, IP address).
	Likewise, generation of all parameters is optional, as is generation
	of the field itself. Also, the field's content is not verified; an
	intermediary can claim certain actions (e.g., sending a request over
	an encrypted channel) but fail to actually do that.
	5. References
	5.1. Normative References
	[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
	Semantics", draft-ietf-httpbis-semantics-19 (work in
	progress), September 2021.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.
	[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
	"Transport Layer Security (TLS) Application-Layer Protocol
	Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
	July 2014, <https://www.rfc-editor.org/info/rfc7301>.
	[RFC8126] 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>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
	Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
	<https://www.rfc-editor.org/info/rfc8446>.
	[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
	Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
	January 2019, <https://www.rfc-editor.org/info/rfc8499>.
	[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
	Lawrence, "Extended DNS Errors", RFC 8914,
	DOI 10.17487/RFC8914, October 2020,
	<https://www.rfc-editor.org/info/rfc8914>.
	[STRUCTURED-FIELDS]
	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>.
	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
	Detection in Content Delivery Networks (CDNs)", RFC 8586,
	DOI 10.17487/RFC8586, April 2019,
	<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
	Mark Nottingham
	Fastly
	Prahran
	Australia
	Email: mnot@mnot.net
	URI: https://www.mnot.net/
	Piotr Sikora
	Google
	Email: piotrsikora@google.com
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