



Network Working Group                                      M. Nottingham
Internet-Draft                                             July 22, 2017
Intended status: Standards Track
Expires: January 23, 2020


                         Site-Wide HTTP Headers
                 draft-nottingham-site-wide-headers-02

Abstract

   This document specifies an alternative way for Web sites to send HTTP
   response header fields that apply to an entire origin, to improve
   efficiency.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 23, 2020.

Copyright Notice

   Copyright (c) 2017 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
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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Selecting Site-Wide Headers . . . . . . . . . . . . . . .   3
     1.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   2.  Server Operation  . . . . . . . . . . . . . . . . . . . . . .   5
   3.  User Agent Operation  . . . . . . . . . . . . . . . . . . . .   6
     3.1.  The "SH" HTTP Request Header Field  . . . . . . . . . . .   6
     3.2.  The "HS" HTTP Response Header Field . . . . . . . . . . .   7
   4.  The "site-headers" well-known URI . . . . . . . . . . . . . .   7
     4.1.  The "text/site-headers" Media Type  . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Injecting Headers . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Differing Views of Headers  . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   HTTP response headers are being used for an increasing amount of
   metadata that applies to an entire Web site (i.e., the entire origin,
   as per [RFC6454]).

   For example, "Strict-Transport-Security" [RFC6797] and "Public-Key-
   Pins" [RFC7469] both define headers that are explicitly scoped to an
   entire origin, and number of similar headers are under consideration.

   Likewise, some HTTP header fields only sensibly have a single value
   per origin; for example, "Server".

   Furthermore, some headers are used uniformly across an origin.  For
   example, a site might have a homogenous "Content-Security-Policy"
   [W3C.CR-CSP2-20150721] header.

   HTTP/2's HPACK [RFC7541] header compression mechanism was designed to
   reduce bandwidth usage for often-repeated headers, both in responses
   and requests.  However, it limits the amount of compression contents
   usable for a connection (by default, 4K), and some sites are
   beginning to exceed this limit, thereby reducing the efficiency of
   HPACK itself.

   For example, it is not uncommon for a CSP response header field to
   exceed 1K (and has been observed to be greater than 3K on popular
   sites).  This forces site administrators to make an awkward choice;



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   put the large header in the HPACK table, thereby crowding out other
   headers, or omit it, requiring its full content to be sent on every
   applicable response.

   This document defines a way to specify one or more sets of HTTP
   response header fields in a well-known resource [RFC5785] that, when
   their use is negotiated, are appended to the header blocks of all
   HTTP responses on that site by the user agent.  This allows common
   response headers to be omitted both from on-the-wire responses and
   the HPACK compression table, making both more efficient.

   This approach is preferable to increasing the HTTP/2
   SETTINGS_HEADER_TABLE_SIZE ([RFC7540], Section 6.5.2), because
   increasing that setting incurs a per-connection overhead on the
   server, whereas using the technique documented here does not.

1.1.  Selecting Site-Wide Headers

   Only certain header fields are suitable for being set for an entire
   origin.  Therefore, a header field MUST be listed below, or its field
   name MUST start with the characters "site-" (case insensitive) to be
   usable as a site-wide header.

   The whitelisted field names are:

   o  Access-Control-Allow-Origin

   o  Alt-Svc

   o  Content-Security-Policy

   o  P3P

   o  Public-Key-Pins

   o  Public-Key-Pins-Report-Only

   o  Server

   o  Strict-Transport-Security

   Note that inclusion in this list does not imply that a header field
   is always site-wide.

   Future specifications SHOULD NOT update this whitelist; instead, they
   SHOULD use the "site-" prefix.





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1.2.  Example

   If a user agent has a fresh copy of the well-known resource for an
   origin (see Section 4) (e.g., because it performed a GET, or HTTP/2
   Server Push was used):

   HTTP/1.1 200 OK
   Content-Type: text/site-headers
   Cache-Control: max-age=3600
   ETag: "abc123"
   Content-Length: 284

   Strict-Transport-Security: max-age=15768000 ; includeSubDomains
   Server: Apache/2.4.7 (Ubuntu)
   Public-Key-Pins: max-age=604800;
     pin-sha256="ZitlqPmA9wodcxkwOW/c7ehlNFk8qJ9FsocodG6GzdjNM=";
     pin-sha256="XRXP987nz4rd1/gS2fJSNVfyrZbqa00T7PeRXUPd15w=";
     report-uri="/lib/key-pin.cgi"
   Site-Foo: bar

   and the user agent makes a subsequent request:

   GET /images/foo.jpg HTTP/1.1
   Host: www.example.com
   SH: "abc123"

   That indicates that the user agent has processed the well-known
   resource (because the "SH" header field is present, and its value
   matches the current value of the "ETag" of the well-known resource).
   Therefore, the server can omit the nominated response header fields
   on the wire, replacing them with the "HS" response header field,
   whose value is the same as that of the "SH" field:

   HTTP/1.1 200 OK
   Content-Type: image/jpeg
   Vary: SH, Accept-Encoding
   Cache-Control: max-age=3600
   Transfer-Encoding: chunked
   HS: "abc123"

   Upon receipt of that response, the user agent will consider it
   equivalent to:









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   HTTP/1.1 200 OK
   Content-Type: image/jpeg
   Vary: SH, Accept-Encoding
   Cache-Control: max-age=3600
   Connection: close
   Strict-Transport-Security: max-age=15768000 ; includeSubDomains
   Server: Apache/2.4.7 (Ubuntu)
   Public-Key-Pins: max-age=604800;
     pin-sha256="ZitlqPmA9wodcxkwOW/c7ehlNFk8qJ9FsocodG6GzdjNM=";
     pin-sha256="XRXP987nz4rd1/gS2fJSNVfyrZbqa00T7PeRXUPd15w=";
     report-uri="/lib/key-pin.cgi"
   Site-Foo: bar

   If a request omits the "SH" header field, or its field-value does not
   match the current "ETag" of the well-known resource, all of the
   header fields above will be sent by the server in the response, and
   "HS" will not be sent.

1.3.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document uses the following ABNF rules from [RFC7230]: "OWS",
   "CRLF", "header-field".  From [RFC7232]: "entity-tag".

2.  Server Operation

   When a server wishes to use site-wide HTTP headers, it places a file
   in the format specified in Section 4.1 at the well-known URI
   specified in Section 4.  That file SHOULD NOT contain header fields
   not allowed by Section 1.1.

   Then, when a request has a "SH" request header field (as per
   Section 3.1) whose value matches the current ETag of the well-known
   resource, the set of response header fields in the payload of the
   well-known resource are omitted from the corresponding response, and
   the "HS" response header field is sent with the same value as the
   "SH" request header field.

   Servers MUST include "SH" in the field-value of the "Vary" response
   header field for all cacheable (as per [RFC7234]) responses of
   resources that behave in this manner, whether or not headers have
   been actually appended.  This assures correct cache operation, and
   also advertises support for this specification.





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   Servers MAY use HTTP/2 Server Push ([RFC7540], Section 8.2) to
   proactively send the well-known resource to user agents (e.g., if
   they emit "SH: *", indicating that they do not have a fresh copy of
   the well-known resource).

3.  User Agent Operation

   User agents that support this specification SHOULD always emit a "SH"
   header field in requests.

   When a valid representation of the well-known resource is held (as
   defined in Section 4), its value will be its "ETag".  When one is not
   (e.g., because it has not been requested, the one held is
   syntactically invalid, or it is stale, as per [RFC7234]), its value
   is "*" (unquoted).

   When an "ETag" is sent and the response contains the "HS" response
   header field (see Section 3.2), user agents MUST confirm that the
   value of the "HS" response header is character-for-character
   identical (after removing leading and trailing whitespace) to that of
   the "SH" request header field it sent.  If it is not, the response
   MUST be considered invalid and MUST NOT be used; the user agent MAY
   retry the request without the "SH" request header field if its method
   was safe, MAY attempt to re-fetch the well-known location beforehand,
   and MAY take alternative recovery strategies.

   If the values match, the user agent MUST append the contents of the
   well-known resource that are currently held to be appended to the
   response headers received, but MUST NOT include any headers not
   allowed by Section 1.1.

3.1.  The "SH" HTTP Request Header Field

   The "SH" HTTP request header field indicates that the user agent has
   a fresh (as per [RFC7234]) copy of the well-known resource (see
   Section 4) for the request's origin ([RFC6454]).

   SH = "*" / entity-tag

   Its value is the "entity-tag" [RFC7232] of the freshest valid well-
   known location response held by the user agent.  If none is held, it
   should be "*" (without quotes).

   For example:

   SH: "abc123"
   SH: *




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3.2.  The "HS" HTTP Response Header Field

   The "HS" HTTP response header field indicates that the server has
   chosen to omit the headers in the well-known resource's response that
   shares its "ETag" with the field value.

   HS = entity-tag

   Its value is the "entity-tag" [RFC7232] of the well-known response
   whose headers are being used, and MUST match that received in the
   "SH" header field of the request.

   For example:

   HS: "abc123"

4.  The "site-headers" well-known URI

   The well-known URI [RFC5785] "site-headers" is a resource that, when
   fetched, returns a representation in the "text/site-headers" format
   (see Section 4.1).

   Its media type SHOULD be generated as "text/site-headers", although
   user agents SHOULD NOT reject responses with other types
   (particularly, "application/octet-stream" and "text/plain").

   Its representation MUST contain an "ETag" response header [RFC7232].

   User agents SHOULD NOT consider it valid if it fails to parse, but
   MAY attempt to recover from errors in a manner similar to how headers
   are normally handled.

   User agents SHOULD consider it to be valid for its freshness lifetime
   (as per [RFC7234]).  If it does not have an explicit freshness
   lifetime, they SHOULD consider it to have a heuristic freshness
   lifetime of 120 seconds.

4.1.  The "text/site-headers" Media Type

   The "text/site-headers" media type is used to indicate that a file
   contains a set of HTTP header fields, as defined in [RFC7230],
   Section 3.

   site-headers = OWS *( header-field CRLF ) OWS

   As in HTTP itself, implementations need to be forgiving about line
   endings; specifically, bare CR MUST be considered to be a line
   ending.



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   For example:

   Strict-Transport-Security: max-age=15768000 ; includeSubDomains
   Server: Apache/2.4.7 (Ubuntu)
   Public-Key-Pins: max-age=604800;
     pin-sha256="ZitlqPmA9wodcxkwOW/c7ehlNFk8qJ9FsocodG6GzdjNM=";
     pin-sha256="XRXP987nz4rd1/gS2fJSNVfyrZbqa00T7PeRXUPd15w=";
     report-uri="/lib/key-pin.cgi"
   Content-Security-Policy: default-src 'self'; img-src 'self'
     *.staticflickr.com; frame-ancestors 'none';
     report-uri https://mnot.report-uri.io/r/default/csp/enforce

   Note that the "Public-Key-Pins" and "Content-Security-Policy" header
   fields are line-folded; as in HTTP, this form of header is deprecated
   in this format, and SHOULD NOT be used (except in documentation, as
   we see here).

5.  IANA Considerations

   TBD

6.  Security Considerations

6.1.  Injecting Headers

   Site-wide headers allow a single resource to inject HTTP response
   headers for an entire origin.  Accordingly, the ability to write to
   that resource needs to be carefully controlled by the origin server.

6.2.  Differing Views of Headers

   Because headers sent via this mechanism will not be seen by user
   agents and intermediaries that do not implement this specification,
   they will potentially have a different view of the response headers.

7.  References

7.1.  Normative References

   [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>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <https://www.rfc-editor.org/info/rfc5785>.



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   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/info/rfc7232>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

7.2.  Informative References

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <https://www.rfc-editor.org/info/rfc6797>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <https://www.rfc-editor.org/info/rfc7469>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/info/rfc7541>.

   [W3C.CR-CSP2-20150721]
              West, M., Barth, A., and D. Veditz, "Content Security
              Policy Level 2", World Wide Web Consortium CR CR-
              CSP2-20150721, July 2015,
              <http://www.w3.org/TR/2015/CR-CSP2-20150721>.







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Author's Address

   Mark Nottingham

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













































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