HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                               P. Sikora
Expires: September 2, 2020 February 12, 2021                                        Google
                                                           March 1,
                                                         August 11, 2020

              The Proxy-Status HTTP Response Header Field
                   draft-ietf-httpbis-proxy-status-01
                   draft-ietf-httpbis-proxy-status-02

Abstract

   This document defines the Proxy-Status HTTP header field to convey the
   details of intermediary handling of responses, 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 September 2, 2020. February 12, 2021.

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  The Proxy-Status HTTP Header Field . . . . . . . . . . . . . . . . .   4
     2.1.  Proxy-Status Parameters . . . . . . . . . . . . . . . . .   5
       2.1.1.  origin  .  next-hop  . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.2.  fwd-protocol  next-protocol . . . . . . . . . . . . . . . . . . . .   6   5
       2.1.3.  error . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.1.4.  details . . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Defining New Proxy-Status Parameters  . . . . . . . . . .   7
     2.3.  Proxy Error Types . . . . . . . . . . . . . . . . . . . .   7
       2.2.1.   8
       2.3.1.  DNS Timeout . . . . . . . . . . . . . . . . . . . . .   7
       2.2.2.   8
       2.3.2.  DNS Error . . . . . . . . . . . . . . . . . . . . . .   7
       2.2.3.   8
       2.3.3.  Destination Not Found . . . . . . . . . . . . . . . .   7
       2.2.4.   8
       2.3.4.  Destination Unavailable . . . . . . . . . . . . . . .   8
       2.2.5.   9
       2.3.5.  Destination IP Prohibited . . . . . . . . . . . . . .   8
       2.2.6.   9
       2.3.6.  Destination IP Unroutable . . . . . . . . . . . . . .   8
       2.2.7.   9
       2.3.7.  Connection Refused  . . . . . . . . . . . . . . . . .   8
       2.2.8.   9
       2.3.8.  Connection Terminated . . . . . . . . . . . . . . . .   9
       2.2.9.  10
       2.3.9.  Connection Timeout  . . . . . . . . . . . . . . . . .   9
       2.2.10.  10
       2.3.10. Connection Read Timeout . . . . . . . . . . . . . . .   9
       2.2.11.  10
       2.3.11. Connection Write Timeout  . . . . . . . . . . . . . .   9
       2.2.12.  10
       2.3.12. Connection Limit Reached  . . . . . . . . . . . . . .  10
       2.2.13. HTTP Incomplete Response  . . . . . . . . . . . . . .  10
       2.2.14. HTTP  11
       2.3.13. TLS Protocol Error  . . . . . . . . . . . . . . . . .  10
       2.2.15. HTTP Response Header Block Too Large  . . .  11
       2.3.14. TLS Certificate Error . . . . .  10
       2.2.16. HTTP Response Header Too Large . . . . . . . . . . .  11
       2.2.17. HTTP Response Body Too Large  . . . .
       2.3.15. TLS Alert Received  . . . . . . . .  11
       2.2.18. HTTP Response Transfer-Coding Error . . . . . . . . .  11
       2.2.19.
       2.3.16. HTTP Response Content-Coding Request Error  . . . . . . . . .  12
       2.2.20. HTTP Response Timeout . . . . . . . . . . . . . . . .  12
       2.2.21. TLS Handshake Error
       2.3.17. HTTP Request Denied . . . . . . . . . . . . . . . . .  12
       2.2.22. TLS Untrusted Peer Certificate  .
       2.3.18. HTTP Incomplete Response  . . . . . . . . . .  12
       2.2.23. TLS Expired Peer Certificate . . . .  12
       2.3.19. HTTP Response Header Block Too Large  . . . . . . . .  13
       2.2.24. TLS Unexpected Peer Certificate
       2.3.20. HTTP Response Header Too Large  . . . . . . . . . . .  13
       2.2.25. TLS Missing Proxy Certificate
       2.3.21. HTTP Response Body Too Large  . . . . . . . . . . . .  13
       2.2.26. TLS Rejected Proxy Certificate  . .
       2.3.22. HTTP Response Transfer-Coding Error . . . . . . . . .  14
       2.2.27. TLS
       2.3.23. HTTP Response Content-Coding Error  . . . . . . . . .  14
       2.3.24. HTTP Response Timeout . . . . . . . . . . . . . . . .  14
       2.2.28.
       2.3.25. HTTP Request Error Upgrade Failed . . . . . . . . . . . . . . . . .  14
       2.2.29.
       2.3.26. HTTP Request Denied Protocol Error . . . . . . . . . . . . . . . . .  15
       2.2.30. HTTP Upgrade Failed . .
       2.3.27. Proxy Internal Response . . . . . . . . . . . . . . .  15
       2.2.31.
       2.3.28. Proxy Internal Response Error  . . . . . . . . . . . . . . . .  15
       2.2.32.
       2.3.29. Proxy Internal Configuration Error . . . . . . . . . . . . . . . .  15
       2.2.33.
       2.3.30. Proxy Loop Detected . . . . . . . . . . . . . . . . .  16
     2.3.
     2.4.  Defining New Proxy Error Types  . . . . . . . . . . . . .  16
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  18
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18  19

1.  Introduction

   HTTP intermediaries - 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 and then forward their responses back to clients.  However, if
   an error occurs, occurs before a response is obtained from upstream, the
   response is 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 header field to
   allow intermediaries to convey details of their handling of a
   response, Section 2.1 enumerates the kind of information that can be
   conveyed, and Section 2.2 2.3 defines a set of Proxy Error Types error types for use when a
   proxy generates the response.  Section 2.3 explains how to
   define new Proxy Error Types.

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 Headers
   [I-D.ietf-httpbis-header-structure] to specify syntax.  The terms sh-
   param-list, sh-item, sh-string, sh-token sf-
   list, sf-item, sf-string, sf-token, sf-integer and sh-integer 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 Header Field

   The Proxy-Status HTTP response header field allows an intermediary to convey
   additional information about its handling of a response and its
   associated request.

   It is a Structured Headers [I-D.ietf-httpbis-header-structure] List
   of parameterised Tokens:

   Cache-Status [I-D.ietf-httpbis-header-structure]:

   Proxy-Status   = sh-list 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: FooProxy, ExampleCDN

   indicates that this response was handled first by FooAccelerator FooProxy and then
   ExampleCDN.

   Parameters on each member convey additional information about that
   intermediary's handling of the response; see Section 2.1 for defined
   parameters.

   Intermediaries determine when it is appropriate to add the Proxy-
   Status header field to a response.  Some might decide to add it to all
   responses, whereas others might only do so when specifically
   configured to, or when the request contains a header that activates a
   debugging mode.

   When adding a value to the Proxy-Status header field, intermediaries
   SHOULD preserve the existing contents of the header, to allow
   debugging of the entire chain of intermediaries handling the request.

   The list members identify the intermediary that inserted the value,
   and MUST have a type of either sh-string sf-string or sh-token. sf-token.  Depending on
   the deployment, this might be a product or service name (e.g.,
   ExampleProxy or "Example CDN"), a hostname ("proxy-3.example.com"),
   and IP address, or a generated string.

   Each

   Parameters on each member of the list can also have a number of parameters that
   describe convey additional information about that
   intermediary's handling of the request. response; see Section 2.1 for defined
   parameters.  While all of these parameters are OPTIONAL,
   intermediaries are encouraged to provide as much information as
   possible.

   When adding a value to the Proxy-Status field, intermediaries SHOULD
   preserve the existing contents of the field, to allow debugging of
   the entire chain of intermediaries handling the request.

   Proxy-Status MAY be sent in HTTP trailers, but - as with all trailers
   - it might be silently discarded along the path to the user agent, so
   this SHOULD NOT be done unless it is not possible to send it in
   headers.  For example, if an intermediary is streaming a response and
   the upstream connection suddenly terminates, Proxy-Status can be
   appended to the trailers of the outgoing message (since the headers
   have already been sent).

   Note that there are various security considerations for
   intermediaries using the Proxy-Status header field; see Section 4.

   Origin servers MUST NOT generate the Proxy-Status header field.

2.1.  Proxy-Status Parameters

   This section lists parameters that can be used on the members of
   Proxy-Status.

2.1.1.  origin  next-hop

   The "origin" "next-hop" parameter's value is a sh-string sf-string or sh-token sf-token that
   identifies the intermediary or origin server selected (and used, if
   contacted) for this response.  Its contents might be a hostname, IP
   address, or alias.

   This is most useful for gateways (also known as "reverse proxies"),
   since they are often configured to use an origin server other than
   that which appears in the URL, and sometimes they use several origins
   to serve a given site.

   For example:

   Proxy-Status: cdn.example.org; origin=backend.example.org next-hop=backend.example.org

2.1.2.  fwd-protocol  next-protocol

   The "fwd-protocol" "next-protocol" parameter's value is a sh-token indicating indicates the ALPN protocol
   identifier [RFC7301] used by the intermediary to connect to the next
   hop.  This is only applicable when that connection was actually
   established.

   The value MUST be either a sf-token or sf-binary.  If the protocol
   identifier is able to be expressed as a sf-token using UTF-8
   encoding, that form MUST be used.

   For example:

   Proxy-Status: "proxy.example.org"; fwd-protocol=h2 next-protocol=h2

2.1.3.  error

   The "error" parameter's value is a sh-token sf-token that is a Proxy Error
   Type.  When present, it indicates that the response was generated by
   the proxy, not the origin server or any other upstream server.

   Section 2.2 2.3 lists the Proxy Error Types defined in this document; new
   ones can be defined using the procedure outlined in Section 2.3. 2.4.

   For example:

   HTTP/1.1 504 Gateway Timeout
   Proxy-Status: SomeCDN; error=connection_timeout

   indicates that this 504 response was generated by SomeCDN, due to a
   connection timeout when going forward.

   Or:

   HTTP/1.1 429 Too Many Requests
   Proxy-Status: SomeReverseProxy; error=http_request_error

   indicates that this 429 Too Many Requests response was generated by
   the intermediary, not 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) when another status code might be used.

2.1.4.  details

   The "details" parameter's value is a sh-string sf-string containing additional
   information not captured anywhere else.  This can include
   implementation-specific or deployment-specific information.

   For example:

  Proxy-Status: ExampleProxy; error="http_protocol_error";
                details="Malformed response header - space before colon"

2.2.  Proxy Error Types

   This section lists the Proxy Error Types  Defining New Proxy-Status Parameters

   New Proxy-Status Parameters can be defined by this document.
   See registering them in the
   HTTP Proxy-Status Parameters registry.

   Registration requests are reviewed and approved by a Designated
   Expert, as per [RFC8126], Section 2.3 for information about defining 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).

   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]

   See the registry at https://iana.org/assignments/http-proxy-status
   [4] 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.

2.2.1.

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

2.2.2.

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 sh-string sf-string conveying the DNS RCODE that indicates the
         error type.  See [RFC8499], Section 3.

   o  Recommended HTTP status code: 502

2.2.3.

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

2.2.4.

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

2.2.5.

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

2.2.6.

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

2.2.7.

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

2.2.8.

2.3.8.  Connection Terminated

   o  Name: connection_terminated

   o  Description: The intermediary's connection to the next hop was
      closed before any part of the response was received.  If some part
      was received, see http_response_incomplete.

   o  Extra Parameters: None.

   o  Recommended HTTP status code: 502

2.2.9.

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

2.2.10.

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

2.2.11.

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

2.2.12.

2.3.12.  Connection Limit Reached

   o  Name: connnection_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

2.2.13.  HTTP Incomplete Response

2.3.13.  TLS Protocol Error

   o  Name: http_response_incomplete tls_protocol_error

   o  Description: The intermediary received an incomplete response to
      the request from encountered a TLS error when
      communicating with the next hop. hop, either during handshake or
      afterwards.

   o  Extra Parameters: None.

   o  Recommended HTTP status code: 502

2.2.14.  HTTP Protocol

   Note that additional information about the error can be recorded in
   the details parameter (as is the case for all errors).

2.3.14.  TLS Certificate Error

   o  Name: http_protocol_error tls_certificate_error

   o  Description: The intermediary encountered a HTTP protocol an error when communicating with verifying
      the certificate presented by 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

2.2.15.  HTTP Response Header Block Too Large

   Note that additional information about the error can be recorded in
   the details parameter (as is the case for all errors).

2.3.15.  TLS Alert Received

   o  Name: http_response_header_block_size tls_alert_received

   o  Description: The intermediary received a response to TLS alert from the request
      whose header block was considered too large. next
      hop.

   o  Extra Parameters:

      *  header_block_size:  alert_message: a sh-integer indicating how large sf-token containing the
         headers received were.  Note that they might not be complete;
         i.e., applicable description
         string from the intermediary may have discarded or refused TLS Alerts registry.

   o  Recommended HTTP status code: 502

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: a sf-integer containing the generated status code.

      *  status_phrase: a sf-string containing the generated status
         phrase.

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

   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

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

2.3.19.  HTTP Response Header Block Too Large

   o  Name: http_response_header_block_size

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

   o  Extra Parameters:

      *  header_block_size: a 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

2.2.16.

2.3.20.  HTTP Response Header Too Large

   o  Name: http_response_header_size

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

   o  Extra Parameters:

      *  header_name: a sh-string sf-string indicating the name of the header that
         triggered the error.

   o  Recommended HTTP status code: 502

2.2.17.

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: a sh-integer 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

2.2.18.

2.3.22.  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: a sh-token sf-token containing the specific coding that caused
         the error.

   o  Recommended HTTP status code: 502

2.2.19.

2.3.23.  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: a sh-token containing the specific coding that caused
         the error.

   o  Recommended HTTP status code: 502

2.2.20.  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

2.2.21.  TLS Handshake Error

   o  Name: tls_handshake_error

   o  Description: The intermediary encountered an error during TLS
      handshake with the next hop.

   o  Extra Parameters:

      *  alert_message: a sh-token containing the applicable description
         string from the TLS Alerts registry.

   o  Recommended HTTP status code: 502

2.2.22.  TLS Untrusted Peer Certificate

   o  Name: tls_untrusted_peer_certificate

   o  Description: The intermediary received an untrusted peer
      certificate during TLS handshake with the next hop.

   o  Extra Parameters: None.

   o  Recommended HTTP status code: 502

2.2.23.  TLS Expired Peer Certificate

   o  Name: tls_expired_peer_certificate

   o  Description: The intermediary received an expired peer certificate
      during TLS handshake with the next hop.

   o  Extra Parameters: None.

   o  Recommended HTTP status code: 502

2.2.24.  TLS Unexpected Peer Certificate

   o  Name: tls_unexpected_peer_certificate

   o  Description: The intermediary received an unexpected peer
      certificate (e.g., SPKI doesn't match) during the TLS handshake
      with the next hop.

   o  Extra Parameters:

      *  identity: a sh-string containing a comma-separated list of
         Subject Alternative Names from the certificate received from
         the next hop.

      *  sha256: a sh-string containing the hex-encoded SHA-256 of the
         certificate received from the next hop.

      *  spki: a sh-string containing the base64-encoded SHA-256 of the
         Subject Public Key Info (SPKI) from the certificate received
         from the next hop.

   o  Recommended HTTP status code: 502

2.2.25.  TLS Missing Proxy Certificate

   o  Name: tls_missing_proxy_certificate

   o  Description: The next hop requested a client certificate from the
      intermediary during TLS handshake, but it wasn't configured with
      one.

   o  Extra Parameters: None.

   o  Recommended HTTP status code: 500

2.2.26.  TLS Rejected Proxy Certificate

   o  Name: tls_rejected_proxy_certificate

   o  Description: The next hop rejected the client certificate provided
      by the intermediary during TLS handshake.

   o  Extra Parameters: None.

   o  Recommended HTTP status code: 500

2.2.27.  TLS Error

   o  Name: tls_error

   o  Description: The intermediary encountered a TLS error when
      communicating with the next hop.

   o  Extra Parameters:

      *  alert_message: a sh-token containing the applicable description
         string from the TLS Alerts registry.

   o  Recommended HTTP status code: 502

2.2.28.  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. decoding the
      content-coding of the response.

   o  Extra Parameters:

      *  status_code:  coding: a sh-integer sf-token containing the generated status code.

      *  status_phrase: a sh-string containing specific coding that caused
         the generated status
         phrase. error.

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

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

2.2.29. 502

2.3.24.  HTTP Request Denied Response Timeout

   o  Name: http_request_denied http_response_timeout

   o  Description: The intermediary rejected the HTTP request based on
      its configuration and/or policy settings.  The request wasn't
      forwarded to reached a configured time limit
      waiting for the next hop. complete response.

   o  Extra Parameters: None.

   o  Recommended HTTP status code: 400

2.2.30. 504

2.3.25.  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

2.2.31.

2.3.26.  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

   Note that additional information about the error can be recorded in
   the details parameter (as is the case for all errors).

2.3.27.  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:

2.2.32.

2.3.28.  Proxy Internal Error

   o  Name: proxy_internal_error

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

   o  Extra Parameters:

      *  error: a sh-string containing details None

   o  Recommended HTTP status code: 500

   Note that additional information about the error
         condition. can be recorded in
   the details parameter (as is the case for all errors).

2.3.29.  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

2.2.33.

   Note that additional information about the error can be recorded in
   the details parameter (as is the case for all errors).

2.3.30.  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

2.3.

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 a Designated
   Expert, 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  If the value is generic;  Generic types are preferred over vendor-specific, application-specific and application-
      specific or deployment-specific values are discouraged 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).

   Registration requests should use the following template:

   o  Name: [a name for the Proxy Error Type that is matches sh-token] 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]

   See the registry at https://iana.org/assignments/http-proxy-statuses
   [4] https://iana.org/assignments/http-proxy-status
   [5] 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-statuses [5] [6] and populate
   it
   them with the types defined in Section 2.2; see 2.1 and Section 2.3
   respectively; see Section 2.2 and Section 2.4 for its associated
   procedures.

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.

   As a result, care needs to be taken when deciding to generate a
   Proxy-Status header. field.  Note that intermediaries are not required to
   generate a Proxy-Status header 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.

5.  References

5.1.  Normative References

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers Field Values for
              HTTP",
              draft-ietf-httpbis-header-structure-13 draft-ietf-httpbis-header-structure-19 (work in
              progress),
              August 2019. June 2020.

   [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>.

   [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>.

5.2.  Informative References

   [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://iana.org/assignments/http-proxy-statuses https://iana.org/assignments/http-proxy-status

   [5] https://iana.org/assignments/http-proxy-status

   [6] https://iana.org/assignments/http-proxy-statuses

Authors' Addresses

   Mark Nottingham
   Fastly
   made in
   Prahran, VIC
   Australia

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

   Piotr Sikora
   Google

   Email: piotrsikora@google.com