Site-Wide HTTP Headers
draft-nottingham-site-wide-headers-00

Network Working Group                                      M. Nottingham
Internet-Draft                                            August 3, 2016
Intended status: Informational
Expires: February 4, 2017

                         Site-Wide HTTP Headers
                 draft-nottingham-site-wide-headers-00

Abstract

   This document specifies an alternative way for Web sites to send HTTP
   response header fields that apply to large numbers of resources, to
   improve efficiency.

Status of This Memo

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Table of Contents

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

1.  Introduction

   HTTP response headers are being used for an increasing amount of
   metadata that applies to an entire site, or large portions of it.

   For example, "Strict-Transport-Security" [RFC6797] and "Public-Key-
   Pins" [RFC7469] both define headers that are explicitly scoped to an
   entire origin [RFC6454], 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 "Content-Security-Policy"
   [W3C.CR-CSP2-20150721] header that doesn't vary across the site, or
   only varies slightly from resource to resource.

   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), which sites are beginning
   to exceed, thereby reducing the efficiency of HPACK itself.

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   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;
   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 HTTP responses 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.  Example

   If a user agent has a fresh copy of the well-known resource for an
   origin (see Section 4), because either 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: 1234

   # a
   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"

   and the user agent makes the request:

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

   this indicates that the user agent has processed the well-known
   resource, and therefore that the server can omit the nominated

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   response header fields on the wire, instead referring to them with
   the "HS" response header field:

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

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

   HTTP/1.1 200 OK
   Content-Type: image/jpeg
   Vary: SM, 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"

   If a request omits the "SM" 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.

1.2.  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 [RFC5234]: "DQUOTE",
   "ALPHA".  From [RFC7230]: "OWS", "RWS", "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.

   Then, when a request has a "SM" request header field (as per
   Section 3.1) that matches the current ETag of the well-known
   resource, the set of response header fields referred to by the "HS"

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   response header field (see Section 2.2) for the requested resource
   are omitted from the corresponding response.

   Servers SHOULD include "SM" 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.

   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 "SM: *", indicating that they do not have a fresh copy of
   the well-known resource).

2.1.  Selecting Site-Wide Headers

   Because this mechanism effectively hides response header fields from
   intermediaries that do not implement it, care ought to be take in
   selecting the headers to use it upon.

   For example, the "Cache-Control" and "Vary" headers are poor
   candidates, because they are often used by intermediaries for HTTP
   caching [RFC7234].

   Likewise, HTTP/1 headers that affect message framing and connection
   behaviour (e.g., "Content-Length", "Transfer-Encoding", "Connection")
   MUST NOT be included in the well-known resource.

2.2.  The "HS" HTTP Response Header Field

   The "HS" HTTP response header field indicates the header set in the
   well-known location file (see Section 4.1) that should be applied to
   the response it occurs within.

   HS = DQUOTE 1*ALPHA DQUOTE

   For example:

   HS: "foo"

3.  User Agent Operation

   User agents that support this specification SHOULD always emit a "SM"
   header field in requests, carrying either the "ETag" of the well-
   known resource currently held for the origin, or "*" to indicate that
   they support this specification, but do not have a fresh (as per
   [RFC7234]) copy of it.

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   User agents might discover that an origin supports this specification
   when it returns a response containing the "HS" response header field,
   or they might learn of it when the well-known location's current
   contents are sent via a HTTP/2 Server Push.

   In either case, user agents SHOULD send a "SM" request header field
   on all requests to such an origin.

   Upon receiving a response to such a request containing the "HS"
   response header field, user agents MUST locate the header-set
   referred to by its field-value in the stored well-known response,
   remove any surrounding white space, and append it to the response
   headers, stripping the "HS" response header field.

   If the corresponding header-set cannot be found in the well-known
   location, the response MUST be considered invalid and MUST NOT be
   used; the user agent MAY retry the request without the "SM" request
   header field if its method was safe, or may take alternative recovery
   strategies.

3.1.  The "SM" HTTP Request Header Field

   The "SM" 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]).

   SM = "*" / 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:

   SM: "abc123"
   SM: *

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

   The well-known URI [RFC5785] "site-headers" is a resource that, when
   fetched, returns a file 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].

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   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 60 seconds.

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

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

   site-headers = 1*( header-header header-set )
   header-header = "#" 1*RWS set-name OWS CRLF
   set-name = 1*ALPHA
   header-set = OWS *( header-field CRLF ) OWS

   Each set of HTTP header fields is started by a header-header, which
   is indicated by an octothorp ("#") followed by the name of the header
   set.  The following lines, up until the next line beginning with an
   octothorp or the end of the file are considered to be the header-
   set's contents.

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

   For example:

   # foo
   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"
   # bar
   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

   This file specifies two sets of HTTP headers, "foo" and "bar".  Note
   that the "Public-Key-Pins" and "Content-Security-Policy" header

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   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).

4.1.1.  Parsing "text/site-headers"

   Given a stream of Unicode characters:

   1.   Let "header-sets" be an empty mapping.

   2.   Consume all characters from up to and including the first
        octothorp ("#").

   3.   Consume all "WSP" characters.

   4.   Let "set-name" be all characters up to but not including the
        next "WSP", "CR" or "LF".

   5.   Consume all "WSP", "CR" and "LF characters".

   6.   Let "header-set" be all characters up to but not including the
        next "CR" or "LF" character followed by an octothorp ("#"), or
        the end of the file.

   7.   Trim all "WSP" from the end of "header-set".

   8.   Let the value of the "set-name" entry in "header-sets" be
        "header-set" (removing any existing value).

   9.   If there is more "input", return to step 2.

   10.  Otherwise, return "header-sets".

   This returns a mapping of "set-name" to a HTTP "header-set", as
   defined in [RFC7230], Section 3.  It SHOULD be parsed as defined
   there.

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.

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6.2.  Inappropriate Headers

   As noted in Section 2.1, there are a variety of HTTP response headers
   which are inappropriate for use as site-wide headers, and some (e.g.,
   "Content-Length") can cause both interoperability and security
   issues.

6.3.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

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

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454, DOI
              10.17487/RFC6454, December 2011,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7232>.

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   [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,
              <http://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,
              <http://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, <http://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,
              <http://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,
              <http://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>.

Author's Address

   Mark Nottingham

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

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