Site-Wide HTTP Headers
draft-nottingham-site-wide-headers-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Mark Nottingham | ||
| Last updated | 2016-08-03 | ||
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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
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 4, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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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