The Proxy-Status HTTP Response Header Field
draft-ietf-httpbis-proxy-status-08

HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                               P. Sikora
Expires: 16 April 2022                                            Google
                                                         13 October 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/
   (https://lists.w3.org/Archives/Public/ietf-http-wg/).

   Working Group information can be found at https://httpwg.org/
   (https://httpwg.org/); source code and issues list for this draft can
   be found at https://github.com/httpwg/http-extensions/labels/proxy-
   status (https://github.com/httpwg/http-extensions/labels/proxy-
   status).

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 16 April 2022.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  The Proxy-Status HTTP Field . . . . . . . . . . . . . . . . .   4
     2.1.  Proxy-Status Parameters . . . . . . . . . . . . . . . . .   6
       2.1.1.  error . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.1.2.  next-hop  . . . . . . . . . . . . . . . . . . . . . .   8
       2.1.3.  next-protocol . . . . . . . . . . . . . . . . . . . .   8
       2.1.4.  received-status . . . . . . . . . . . . . . . . . . .   8
       2.1.5.  details . . . . . . . . . . . . . . . . . . . . . . .   9
     2.2.  Defining New Proxy-Status Parameters  . . . . . . . . . .   9
     2.3.  Proxy Error Types . . . . . . . . . . . . . . . . . . . .  10
       2.3.1.  DNS Timeout . . . . . . . . . . . . . . . . . . . . .  10
       2.3.2.  DNS Error . . . . . . . . . . . . . . . . . . . . . .  10
       2.3.3.  Destination Not Found . . . . . . . . . . . . . . . .  11
       2.3.4.  Destination Unavailable . . . . . . . . . . . . . . .  11
       2.3.5.  Destination IP Prohibited . . . . . . . . . . . . . .  11
       2.3.6.  Destination IP Unroutable . . . . . . . . . . . . . .  12
       2.3.7.  Connection Refused  . . . . . . . . . . . . . . . . .  12
       2.3.8.  Connection Terminated . . . . . . . . . . . . . . . .  12
       2.3.9.  Connection Timeout  . . . . . . . . . . . . . . . . .  13
       2.3.10. Connection Read Timeout . . . . . . . . . . . . . . .  13
       2.3.11. Connection Write Timeout  . . . . . . . . . . . . . .  13
       2.3.12. Connection Limit Reached  . . . . . . . . . . . . . .  14
       2.3.13. TLS Protocol Error  . . . . . . . . . . . . . . . . .  14
       2.3.14. TLS Certificate Error . . . . . . . . . . . . . . . .  14
       2.3.15. TLS Alert Received  . . . . . . . . . . . . . . . . .  15
       2.3.16. HTTP Request Error  . . . . . . . . . . . . . . . . .  15
       2.3.17. HTTP Request Denied . . . . . . . . . . . . . . . . .  16
       2.3.18. HTTP Incomplete Response  . . . . . . . . . . . . . .  16
       2.3.19. HTTP Response Header Section Too Large  . . . . . . .  16
       2.3.20. HTTP Response Header Field Line Too Large . . . . . .  17
       2.3.21. HTTP Response Body Too Large  . . . . . . . . . . . .  17

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       2.3.22. HTTP Response Trailer Section Too Large . . . . . . .  18
       2.3.23. HTTP Response Trailer Field Line Too Large  . . . . .  18
       2.3.24. HTTP Response Transfer-Coding Error . . . . . . . . .  18
       2.3.25. HTTP Response Content-Coding Error  . . . . . . . . .  19
       2.3.26. HTTP Response Timeout . . . . . . . . . . . . . . . .  19
       2.3.27. HTTP Upgrade Failed . . . . . . . . . . . . . . . . .  19
       2.3.28. HTTP Protocol Error . . . . . . . . . . . . . . . . .  20
       2.3.29. Proxy Internal Response . . . . . . . . . . . . . . .  20
       2.3.30. Proxy Internal Error  . . . . . . . . . . . . . . . .  20
       2.3.31. Proxy Configuration Error . . . . . . . . . . . . . .  21
       2.3.32. Proxy Loop Detected . . . . . . . . . . . . . . . . .  21
     2.4.  Defining New Proxy Error Types  . . . . . . . . . . . . .  21
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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.

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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.

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   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

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   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.

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   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.

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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
   (https://www.iana.org/assignments/tls-extensiontype-values/tls-
   extensiontype-values.xhtml#alpn-protocol-ids)).  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

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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:

   *  Community feedback

   *  If the value is sufficiently well-defined

   *  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).

   *  Parameter names should not conflict with registered extra
      parameters in the Proxy Error Type Registry.

   Registration requests should use the following template:

   *  Name: [a name for the Proxy-Status Parameter that matches key]

   *  Description: [a description of the parameter semantics and value]

   *  Reference: [to a specification defining this parameter; optional]

   See the registry at https://iana.org/assignments/http-proxy-status
   (https://iana.org/assignments/http-proxy-status) for details on where
   to send registration requests.

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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

   *  Name: dns_timeout

   *  Description: The intermediary encountered a timeout when trying to
      find an IP address for the next hop hostname.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 504

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.2.  DNS Error

   *  Name: dns_error

   *  Description: The intermediary encountered a DNS error when trying
      to find an IP address for the next hop hostname.

   *  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].

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

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2.3.3.  Destination Not Found

   *  Name: destination_not_found

   *  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.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 500

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.4.  Destination Unavailable

   *  Name: destination_unavailable

   *  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.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 503

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.5.  Destination IP Prohibited

   *  Name: destination_ip_prohibited

   *  Description: The intermediary is configured to prohibit
      connections to the next hop IP address.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: true

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   *  Reference: [this document]

2.3.6.  Destination IP Unroutable

   *  Name: destination_ip_unroutable

   *  Description: The intermediary cannot find a route to the next hop
      IP address.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.7.  Connection Refused

   *  Name: connection_refused

   *  Description: The intermediary's connection to the next hop was
      refused.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.8.  Connection Terminated

   *  Name: connection_terminated

   *  Description: The intermediary's connection to the next hop was
      closed before complete response was received.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

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2.3.9.  Connection Timeout

   *  Name: connection_timeout

   *  Description: The intermediary's attempt to open a connection to
      the next hop timed out.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 504

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.10.  Connection Read Timeout

   *  Name: connection_read_timeout

   *  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.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 504

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.11.  Connection Write Timeout

   *  Name: connection_write_timeout

   *  Description: The intermediary was attempting to write data to a
      connection, but was not able to (e.g., because its buffers were
      full).

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 504

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

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2.3.12.  Connection Limit Reached

   *  Name: connection_limit_reached

   *  Description: The intermediary is configured to limit the number of
      connections it has to the next hop, and that limit has been
      passed.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 503

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.13.  TLS Protocol Error

   *  Name: tls_protocol_error

   *  Description: The intermediary encountered a TLS error when
      communicating with the next hop, either during handshake or
      afterwards.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

   *  Notes: Not appropriate when a TLS alert is received; see
      tls_alert_received

2.3.14.  TLS Certificate Error

   *  Name: tls_certificate_error

   *  Description: The intermediary encountered an error when verifying
      the certificate presented by the next hop.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: true

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   *  Reference: [this document]

2.3.15.  TLS Alert Received

   *  Name: tls_alert_received

   *  Description: The intermediary received a TLS alert from the next
      hop.

   *  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].

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.16.  HTTP Request Error

   *  Name: http_request_error

   *  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.

   *  Extra Parameters:

      -  status-code: an sf-integer containing the generated status
         code.

      -  status-phrase: an sf-string containing the generated status
         phrase.

   *  Recommended HTTP status code: The applicable 4xx status code

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

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   *  Notes: This type helps distinguish between responses generated by
      intermediaries from those generated by the origin.

2.3.17.  HTTP Request Denied

   *  Name: http_request_denied

   *  Description: The intermediary rejected the HTTP request based on
      its configuration and/or policy settings.  The request wasn't
      forwarded to the next hop.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 403

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.18.  HTTP Incomplete Response

   *  Name: http_response_incomplete

   *  Description: The intermediary received an incomplete response to
      the request from the next hop.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.19.  HTTP Response Header Section Too Large

   *  Name: http_response_header_section_size

   *  Description: The intermediary received a response to the request
      whose header section was considered too large.

   *  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.

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   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.20.  HTTP Response Header Field Line Too Large

   *  Name: http_response_header_size

   *  Description: The intermediary received a response to the request
      containing an individual header field line that was considered too
      large.

   *  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.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.21.  HTTP Response Body Too Large

   *  Name: http_response_body_size

   *  Description: The intermediary received a response to the request
      whose body was considered too large.

   *  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.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

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2.3.22.  HTTP Response Trailer Section Too Large

   *  Name: http_response_trailer_section_size

   *  Description: The intermediary received a response to the request
      whose trailer section was considered too large.

   *  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.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.23.  HTTP Response Trailer Field Line Too Large

   *  Name: http_response_trailer_size

   *  Description: The intermediary received a response to the request
      containing an individual trailer field line that was considered
      too large.

   *  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.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.24.  HTTP Response Transfer-Coding Error

   *  Name: http_response_transfer_coding

   *  Description: The intermediary encountered an error decoding the
      transfer-coding of the response.

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   *  Extra Parameters:

      -  coding: an sf-token containing the specific coding (from the
         HTTP Transfer Coding Registry) that caused the error.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.25.  HTTP Response Content-Coding Error

   *  Name: http_response_content_coding

   *  Description: The intermediary encountered an error decoding the
      content-coding of the response.

   *  Extra Parameters:

      -  coding: an sf-token containing the specific coding (from the
         HTTP Content Coding Registry) that caused the error.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.26.  HTTP Response Timeout

   *  Name: http_response_timeout

   *  Description: The intermediary reached a configured time limit
      waiting for the complete response.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 504

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.27.  HTTP Upgrade Failed

   *  Name: http_upgrade_failed

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   *  Description: The HTTP Upgrade between the intermediary and the
      next hop failed.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.28.  HTTP Protocol Error

   *  Name: http_protocol_error

   *  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.

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: false

   *  Reference: [this document]

2.3.29.  Proxy Internal Response

   *  Name: proxy_internal_response

   *  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).

   *  Extra Parameters: None.

   *  Recommended HTTP status code: The most appropriate status code for
      the response

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.30.  Proxy Internal Error

   *  Name: proxy_internal_error

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   *  Description: The intermediary encountered an internal error
      unrelated to the origin.

   *  Extra Parameters: None

   *  Recommended HTTP status code: 500

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.31.  Proxy Configuration Error

   *  Name: proxy_configuration_error

   *  Description: The intermediary encountered an error regarding its
      configuration.

   *  Extra Parameters: None

   *  Recommended HTTP status code: 500

   *  Response only generated by intermediaries: true

   *  Reference: [this document]

2.3.32.  Proxy Loop Detected

   *  Name: proxy_loop_detected

   *  Description: The intermediary tried to forward the request to
      itself, or a loop has been detected using different means (e.g.
      [RFC8586]).

   *  Extra Parameters: None.

   *  Recommended HTTP status code: 502

   *  Response only generated by intermediaries: true

   *  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.

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   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:

   *  Community feedback

   *  If the value is sufficiently well-defined

   *  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).

   *  Extra Parameters should not conflict with registered Proxy-Status
      parameters.

   Registration requests should use the following template:

   *  Name: [a name for the Proxy Error Type that matches sf-token]

   *  Description: [a description of the conditions that generate the
      Proxy Error Type]

   *  Extra Parameters: [zero or more optional parameters, along with
      their allowable type(s)]

   *  Recommended HTTP status code: [the appropriate HTTP status code
      for this entry]

   *  Response only generated by intermediaries: ['true' or 'false']

   *  Reference: [to a specification defining this error type; optional]

   *  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'.

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   See the registry at https://iana.org/assignments/http-proxy-status
   (https://iana.org/assignments/http-proxy-status) 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
   (https://iana.org/assignments/http-proxy-status) 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:

   *  Field name: Proxy-Status

   *  Status: permanent

   *  Specification document(s): [this document]

   *  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

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   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [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/rfc/rfc8941>.

   [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/rfc/rfc2119>.

   [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/rfc/rfc8126>.

   [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/rfc/rfc8499>.

   [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/rfc/rfc8174>.

   [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/rfc/rfc7301>.

   [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/rfc/rfc8914>.

   [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/rfc/rfc8446>.

5.2.  Informative References

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   [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/rfc/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/rfc/rfc8586>.

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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